mlx4_en: Remove redundant refill code on RX

Our RX rings are always full, there is no need to check whether
we need to fill them or not. If we fail to allocate a new socket
buffer, the incoming packet is dropped an the ring remains full.

Signed-off-by: Yevgeny Petrilin <yevgenyp@mellanox.co.il>
Signed-off-by: David S. Miller <davem@davemloft.net>