ptp: Split out PTP_PIN_SETFUNC ioctl code

Continue the ptp_ioctl() cleanup by splitting out the PTP_PIN_SETFUNC ioctl
code into a helper function. Convert to lock guard while at it and remove
the pointless memset of the pd::rsv because nothing uses 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/20250625115133.241503804@linutronix.de


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