ptp: Split out PTP_MASK_EN_SINGLE ioctl code

Finish the ptp_ioctl() cleanup by splitting out the PTP_MASK_EN_SINGLE
ioctl code and removing the remaining local variables and return
statements.

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/20250625115133.364422719@linutronix.de


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