bnx2x: Fix bridged GSO for 57710/57711 chips

It was recently found out that GSO on 57710/57711 was broken, due to packets
being sent without a valid IP checksum.

Commit 057cf65e

 "bnx2x: Fix GSO for 57710/57711 chips" partially fixed this
issue, but failed to set the correct IP checksum when receiving GSO packets
via bridges, as such packets enter bnx2x_tx_split() and the FW flags needed
to calculate IP checksum were erroneously set in the incorrect
buffer descriptor.

This patch re-enables GSO in said scenario for 57710/57711 chips.

Signed-off-by: Yuval Mintz <yuvalmin@broadcom.com>
Signed-off-by: Ariel Elior <ariele@broadcom.com>
Signed-off-by: Eilon Greenstein <eilong@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>