ptp: Return -EINVAL on ptp_clock_register if required ops are NULL

ptp: Return -EINVAL on ptp_clock_register if required ops are NULL

ptp_clock should never be registered unless it stubs one of gettimex64()
or gettime64() and settime64(). WARN_ON_ONCE and error out if either set
of function pointers is null.

For consistency, n_alarm validation is also folded into the
WARN_ON_ONCE.

Suggested-by: Kuniyuki Iwashima <kuniyu@google.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>
Reviewed-by: Harshitha Ramamurthy <hramamurthy@google.com>
Reviewed-by: Vadim Fedorenko <vadim.fedorenko@linux.dev>
Signed-off-by: Tim Hostetler <thostet@google.com>
Acked-by: Richard Cochran <richardcochran@gmail.com>
Link: https://patch.msgid.link/20251104225915.2040080-1-thostet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>