ptp: Split out PTP_ENABLE_PPS ioctl code

Continue the ptp_ioctl() cleanup by splitting out the PTP_ENABLE_PPS
ioctl code into a helper function. Convert to a lock guard while at it.

No functional change intended.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Vadim Fedorenko <vadim.fedorenko@linux.dev>
Link: https://patch.msgid.link/20250625115132.923803136@linutronix.de


Signed-off-by: Jakub Kicinski <kuba@kernel.org>