ice: remove unnecessary CONFIG_ICE_GNSS

CONFIG_ICE_GNSS was added by commit c7ef8221ca7d ("ice: use GNSS subsystem
instead of TTY") as a way to allow the ice driver to optionally support
GNSS features without forcing a dependency on CONFIG_GNSS.

The original implementation of that commit at [1] used IS_REACHABLE. This
was rejected by Olek at [2] with the suggested implementation of
CONFIG_ICE_GNSS.

Eventually after merging, Linus reported a .config which had
CONFIG_ICE_GNSS = y when both GNSS = n and ICE = n. This confused him and
he felt that the config option was not useful, and commented about it at
[3].

CONFIG_ICE_GNSS is defined to y whenever GNSS = ICE. This results in it
being set in cases where both options are not enabled.

The goal of CONFIG_ICE_GNSS is to ensure that the GNSS support in the ice
driver is enabled when GNSS is enabled.

The complaint from Olek about the original IS_REACHABLE was due to the
required IS_REACHABLE checks throughout the ice driver code and the fact
that ice_gnss.c was compiled regardless of GNSS support.

This can be fixed in the Makefile by using ice-$(CONFIG_GNSS) += ice_gnss.o

In this case, if GNSS = m and ICE = y, we can result in some confusing
behavior where GNSS support is not enabled because its not built in. See
[4].

To disallow this, have CONFIG_ICE depend on GNSS || GNSS = n. This ensures
that we cannot enable CONFIG_ICE as builtin while GNSS is a module.

Drop CONFIG_ICE_GNSS, and replace the IS_ENABLED checks for it with
checks for GNSS. Update the Makefile to add the ice_gnss.o object based on
CONFIG_GNSS.

This works to ensure that GNSS support can optionally be enabled, doesn't
have an unnnecessary extra config option, and has Kbuild enforce the
dependency such that you can't accidentally enable GNSS as a module and ICE
as a builtin.

[1] https://lore.kernel.org/intel-wired-lan/20221019095603.44825-1-arkadiusz.kubalewski@intel.com/
[2] https://lore.kernel.org/intel-wired-lan/20221028165706.96849-1-alexandr.lobakin@intel.com/
[3] https://lore.kernel.org/all/CAHk-=wi_410KZqHwF-WL5U7QYxnpHHHNP-3xL=g_y89XnKc-uw@mail.gmail.com/
[4] https://lore.kernel.org/netdev/20230223161309.0e439c5f@kernel.org/

Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Fixes: c7ef8221ca7d ("ice: use GNSS subsystem instead of TTY")
Cc: Arkadiusz Kubalewski <arkadiusz.kubalewski@intel.com>
Cc: Alexander Lobakin <alexandr.lobakin@intel.com>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: Anthony Nguyen <anthony.l.nguyen@intel.com>
Acked-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>