net: qualcomm: rmnet: simplify rmnet_map_get_csum_field()

The checksum fields of the TCP and UDP header structures already
have type __sum16.  We don't support any other protocol headers, so
we can simplify rmnet_map_get_csum_field(), getting rid of the local
variable entirely and just returning the appropriate address.

Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>