bnx2x: Added FW GRO bridging support

Since submit 621b4d66

 the bnx2x driver support FW GRO.
However, when using the device with GRO enabled in bridging
scenarios throughput is very low, as the bridge expects all
incoming packets to be passed with CHECKSUM_PARTIAL -
a demand which is satisfied by the SW GRO implementation,
but was missed in the bnx2x driver implementation (which returned
CHECKSUM_UNNECESSARY).

Now, given that the traffic is supported by FW GRO (TCP/IP),
the bnx2x driver calculates the pseudo checksum by itself,
passing skbs with CHECKSUM_PARTIAL and giving a much better
throughput when receiving GRO traffic.

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>