bnxt_en: Fix compile warnings when CONFIG_INET is not set.

bnxt_gro_skb() has unused variables when CONFIG_INET is not set.  We
really cannot support hardware GRO if CONFIG_INET is not set, so
compile out bnxt_gro_skb() completely and define BNXT_FLAG_GRO to be 0
if CONFIG_INET is not set.  This will effectively always disable
hardware GRO if CONFIG_INET is not set.

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