gve: fix NULL dereference due to missing ptp adjfine

gve: fix NULL dereference due to missing ptp adjfine

Fix NULL dereference due to missing implementation of adjfine, which can
be triggered from usermode as follows:

sudo ./testptp -d /dev/ptp0 -f 0
[  551.943697] BUG: kernel NULL pointer dereference, address: 0000000000000000
[...]
[  552.061946] Call Trace:
[  552.064487]  <TASK>
[  552.066681]  ptp_clock_adjtime+0x1c0/0x2c0
[  552.070874]  ? get_clock_desc+0x6b/0xb0
[  552.074825]  pc_clock_adjtime+0x78/0xc0
[  552.078755]  __do_sys_clock_adjtime+0x85/0x110
[  552.083293]  do_syscall_64+0xea/0x610

Cc: stable@vger.kernel.org
Fixes: acd16380523b ("gve: Add initial PTP device support")
Signed-off-by: Jordan Rhee <jordanrhee@google.com>
Signed-off-by: Harshitha Ramamurthy <hramamurthy@google.com>
Reviewed-by: Vadim Fedorenko <vadim.fedorenko@linux.dev>
Link: https://patch.msgid.link/20260807224315.234152-3-hramamurthy@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>