gve: Avoid freeing NULL pointer

Prevent possible crashes when cleaning up after unsuccessful
initializations.

Fixes: 893ce44d

 ("gve: Add basic driver framework for Compute Engine Virtual NIC")
Signed-off-by: Tao Liu <xliutaox@google.com>
Signed-off-by: Catherine Sully <csully@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>