enic: Bug fix: try harder to fill Rx ring on skb allocation failures

During dev->open(), make sure we get at least one skb on the Rx ring.
Otherwise abort the interface load.  Also, if we get skb allocation
failures in NAPI poll while trying to replenish the ring, try again
later so we don't end up starving out the Rx ring completely.

Signed-off-by: Vasanthy Kolluri <vkolluri@cisco.com>
Signed-off-by: Scott Feldman <scofeldm@cisco.com>
Signed-off-by: David S. Miller <davem@davemloft.net>