bnxt_en: Increment checksum error counter only if NETIF_F_RXCSUM is set.

rx_l4_csum_error is now incremented only when offload is enabled

Signed-off-by: Satish Baddipadige <sbaddipa@broadcom.com>
Signed-off-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>