bnxt_en: Add GRO logic for BCM5731X chips.

Add bnxt_gro_func_5731x() to handle GRO packets for this chip.  The
completion structures used in the new chip have new data to help determine
the header offsets.  The offsets can be off by 4 if the packet is an
internal loopback packet (e.g. from one VF to another VF).  Some additional
logic is added to adjust the offsets if it is a loopback packet.

Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>