selftests: ptp: treat unsupported PHC operations as skip

selftests: ptp: treat unsupported PHC operations as skip

Some PTP hardware clock (PHC) devices may return -EOPNOTSUPP for
operations like settime, adjtime, or adjfreq. This commonly occurs
with timestamp-only PHC implementations that don't support full clock
control.

For background, syzbot previously exposed a crash risk when PTP clock
drivers lacked required callbacks[1]. Subsequent work[2] made callback
presence a registration requirement. As a result, some drivers (like
iwlwifi MVM/MLD[3]) now provide stub callbacks that return -EOPNOTSUPP
for unsupported operations.

When phc_ctl encounters such devices, the "Operation not supported"
error should be treated as a skip (device limitation) rather than a
test failure. This patch:
- Adds [SKIP] output handling in log_test()
- Detects "Operation not supported" from phc_ctl and returns ksft_skip
- Returns ksft_skip if all tests are skipped, preventing false-positive
  results when testing timestamp-only PHC implementations

Link: https://lore.kernel.org/netdev/20251028043216.1971292-1-junjie.cao@intel.com/
Link: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=dfb073d32cac
Link: https://lore.kernel.org/netdev/20251204123204.9316-1-ziyao@disroot.org/
Signed-off-by: Junjie Cao <junjie.cao@intel.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20260126061532.12532-2-junjie.cao@intel.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>