net: macb: Update gem PTP support check

There are currently two checks for PTP functionality - one on GEM
capability and another on the kernel config option. Combine them
into a single function as there's no use case where gem_has_ptp is
TRUE and MACB_USE_HWSTAMP is false.

Signed-off-by: Harini Katakam <harini.katakam@amd.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>