broadcom: fix support for PTP_EXTTS_REQUEST2 ioctl

Commit 7c571ac57d9d ("net: ptp: introduce .supported_extts_flags to
ptp_clock_info") modified the PTP core kernel logic to validate the
supported flags for the PTP_EXTTS_REQUEST ioctls, rather than relying on
each individual driver correctly checking its flags.

The bcm_ptp_enable() function implements support for PTP_CLK_REQ_EXTTS, but
does not check the flags, and does not forward the request structure into
bcm_ptp_extts_locked().

When originally converting the bcm-phy-ptp.c code, it was unclear what
edges the hardware actually timestamped. Thus, no flags were initialized in
the .supported_extts_flags field. This results in the kernel automatically
rejecting all userspace requests for the PTP_EXTTS_REQUEST2 ioctl.

This occurs because the PTP_STRICT_FLAGS is always assumed when operating
under PTP_EXTTS_REQUEST2. This has been the case since the flags
introduction by commit 6138e687c7b6 ("ptp: Introduce strict checking of
external time stamp options.").

The bcm-phy-ptp.c logic never properly supported strict flag validation,
as it previously ignored all flags including both PTP_STRICT_FLAGS and the
PTP_FALLING_EDGE and PTP_RISING_EDGE flags.

Reports from users in the field prove that the hardware timestamps the
rising edge. Encode this in the .supported_extts_flags field. This
re-enables support for the PTP_EXTTS_REQUEST2 ioctl.

Reported-by: James Clark <jjc@jclark.com>
Fixes: 7c571ac57d9d ("net: ptp: introduce .supported_extts_flags to ptp_clock_info")
Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Reviewed-by: Vadim Fedorenko <vadim.fedorenko@linux.dev>
Acked-by: Richard Cochran <richardcochran@gmail.com>
Reviewed-by: Kory Maincent <kory.maincent@bootlin.com>
Tested-by: James Clark <jjc@jclark.com>
Link: https://patch.msgid.link/20250918-jk-fix-bcm-phy-supported-flags-v1-2-747b60407c9c@intel.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>