bnxt_en: Refactor tunneled hardware GRO logic.

The 2 GRO functions to set up the hardware GRO SKB fields for 2
different hardware chips have practically identical logic for
tunneled packets.  Refactor the logic into a separate bnxt_gro_tunnel()
function that can be used by both functions.

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