net: qualcomm: rmnet: Add support for GRO

Add gro_cells so that rmnet devices can call gro_cells_receive
instead of netif_receive_skb.

Signed-off-by: Subash Abhinov Kasiviswanathan <subashab@codeaurora.org>
Cc: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>