gve: Check TX QPL was actually assigned

Correctly check the TX QPL was assigned and unassigned if
other steps in the allocation fail.

Fixes: f5cedc84

 (gve: Add transmit and receive support)
Signed-off-by: Catherine Sullivan <csully@google.com>
Signed-off-by: David Awogbemila <awogbemila@google.com>
Acked-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>