net: rmnet_data: Call the GRO handler for TCP packets only

While running a UDP test using gro handler, there was a slight
increase in MIPS observed. This is because all UDP packets passed
through napi_gro_receive need to be checked if it is an encapsulated
UDP packet which could actually be coalesced and processed further.

As of now, clients of rmnet_data do not support this and we can
save MIPS on this by passing only TCP packets through the GRO
handler. All non TCP packets will directly be passed directly to
the network stack. This also helps us to avoid an atomic operation
unnecessarily when GRO path is not exercised.

There is 1% savings in MIPS observed when using a single stream
UDP DL connection at 300Mbps.

Change-Id: Ie601000de21afeacfca93f23117aeb0f7cefda98
Signed-off-by: Subash Abhinov Kasiviswanathan <subashab@codeaurora.org>