gve: Propagate error codes to caller

If 'gve_probe()' fails, we should propagate the error code, instead of
hard coding a -ENXIO value.
Make sure that all error handling paths set a correct value for 'err'.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Reviewed-by: Catherine Sullivan <csully@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>