gve: Return error for unknown admin queue command

In gve_adminq_issue_cmd(), return -EINVAL instead of 0 when an unknown
admin queue command opcode is encountered.

This prevents the function from silently succeeding on invalid input
and prevents undefined behavior by ensuring the function fails gracefully
when an unrecognized opcode is provided.

These changes improve error handling.

Signed-off-by: Alok Tiwari <alok.a.tiwari@oracle.com>
Link: https://patch.msgid.link/20250616054504.1644770-2-alok.a.tiwari@oracle.com


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