net: qualcomm: rmnet: Add support for GSO

Real devices may support scatter gather(SG), so enable SG on rmnet
devices to use GSO. GSO reduces CPU cycles by 20% for a rate of
146Mpbs for a single stream TCP connection.

Signed-off-by: Subash Abhinov Kasiviswanathan <subashab@codeaurora.org>
Signed-off-by: David S. Miller <davem@davemloft.net>