ptp: Split out PTP_CLOCK_GETCAPS ioctl code

ptp_ioctl() is an inpenetrable letter soup with a gazillion of case (scope)
specific variables defined at the top of the function and pointless breaks
and gotos.

Start cleaning it up by splitting out the PTP_CLOCK_GETCAPS ioctl code into
a helper function. Use a argument pointer with a single sparse compliant
type cast instead of proliferating the type cast all over the place.

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/20250625115132.733409073@linutronix.de


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