Add logic to reserve default rings at driver open time if none was
reserved during probe time.  This will happen when the PF driver did
not provision minimum rings to the VF, due to more limited resources.

Driver open will only succeed if some minimum rings can be reserved.
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>