dpll: fix device-id-get and pin-id-get to return errors properly

The device-id-get and pin-id-get handlers were ignoring errors from
the find functions and sending empty replies instead of returning
error codes to userspace.

When dpll_device_find_from_nlattr() or dpll_pin_find_from_nlattr()
returned an error (e.g., -EINVAL for "multiple matches" or -ENODEV
for "not found"), the handlers checked `if (!IS_ERR(ptr))` and
skipped adding the device/pin handle to the message, but then still
sent the empty message as a successful reply.

This caused userspace tools to receive empty responses with id=0
instead of proper netlink errors with extack messages like
"multiple matches".

The bug is visible via strace, which shows the kernel sending TWO
netlink messages in response to a single request:

1. Empty reply (20 bytes, just header, no attributes):
recvfrom(3, [{nlmsg_len=20, nlmsg_type=dpll, nlmsg_flags=0, ...},
{cmd=0x7, version=1}], ...)

2. NLMSG_ERROR ACK with extack (because of NLM_F_ACK flag):
recvfrom(3, [{nlmsg_len=60, nlmsg_type=NLMSG_ERROR,
nlmsg_flags=NLM_F_CAPPED|NLM_F_ACK_TLVS, ...},
[{error=0, msg={...}},
[{nla_type=NLMSGERR_ATTR_MSG}, "multiple matches"]]], ...)

The C YNL library parses the first message, sees an empty response,
and creates a result object with calloc() which zero-initializes all
fields, resulting in id=0.

The Python YNL library parses both messages and displays the extack
from the second NLMSG_ERROR message.

Fix by checking `if (IS_ERR(ptr))` first and returning the error
code immediately, so that netlink properly sends only NLMSG_ERROR with
the extack message to userspace. After this fix, both C and Python
YNL tools receive only the NLMSG_ERROR and behave consistently.

This affects:
- DPLL_CMD_DEVICE_ID_GET: now properly returns error when multiple
devices match the criteria (e.g., same module-name + clock-id)
- DPLL_CMD_PIN_ID_GET: now properly returns error when multiple pins
match the criteria (e.g., same module-name)

Before fix:
$ dpll pin id-get module-name ice
0 (wrong - should be error, there are 17 pins with module-name "ice")

After fix:
$ dpll pin id-get module-name ice
Error: multiple matches
(correct - kernel reports the ambiguity via extack)

Fixes: 9d71b54b65b1 ("dpll: netlink: Add DPLL framework base functions")
Signed-off-by: Petr Oros <poros@redhat.com>
Reviewed-by: Ivan Vecera <ivecera@redhat.com>
Link: https://patch.msgid.link/20251024190733.364101-1-poros@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>