mlx4_en: Fix build warning in mlx4_en_create_rx_ring.

drivers/net/mlx4/en_rx.c: In function ‘mlx4_en_create_rx_ring’:
drivers/net/mlx4/en_rx.c:305: warning: label ‘err_map’ defined but not used

Signed-off-by: David S. Miller <davem@davemloft.net>