gve: Implement gettimex64 with -EOPNOTSUPP

gve: Implement gettimex64 with -EOPNOTSUPP

gve implemented a ptp_clock for sole use of do_aux_work at this time.
ptp_clock_gettime() and ptp_sys_offset() assume every ptp_clock has
implemented either gettimex64 or gettime64. Stub gettimex64 and return
-EOPNOTSUPP to prevent NULL dereferencing.

Fixes: acd16380523b ("gve: Add initial PTP device support")
Reported-by: syzbot+c8c0e7ccabd456541612@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=c8c0e7ccabd456541612
Signed-off-by: Tim Hostetler <thostet@google.com>
Reviewed-by: Harshitha Ramamurthy <hramamurthy@google.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>
Signed-off-by: Joshua Washington <joshwash@google.com>
Link: https://patch.msgid.link/20251029184555.3852952-2-joshwash@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>