selftests: ptp: use KSFT_SKIP exit code for skip scenarios

selftests: ptp: use KSFT_SKIP exit code for skip scenarios

The kselftest framework defines KSFT_SKIP=4 as the standard exit code
for skipped tests. However, phc.sh currently uses a mix of 'exit 0' and
'exit 1' to indicate skip conditions, which can confuse test harnesses
and CI systems.

This patch introduces ksft_skip=4 variable and unifies all skip exit
paths to use 'exit $ksft_skip', consistent with other selftests like
net/lib.sh and net/fib_nexthops.sh.

Signed-off-by: Junjie Cao <junjie.cao@intel.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20260126061532.12532-1-junjie.cao@intel.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>