bnx2x: fix UDP checksum for 57710/57711.

Since commit 86564c3f

 "bnx2x: Remove many sparse warnings" UDP
csum offload is broken for 57710/57711. Fix return value.

Signed-off-by: Dmitry Kravkov <dmitry@broadcom.com>
CC: Ariel Elior <ariele@broadcom.com>
CC: Yuval Mintz <yuvalmin@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>