net: qualcomm: rmnet: Add support for TX checksum offload

TX checksum offload applies to TCP / UDP packets which are not
fragmented using the MAPv4 checksum trailer. The following needs to be
done to have checksum computed in hardware -

1. Set the checksum start offset and inset offset.
2. Set the csum_enabled bit
3. Compute and set 1's complement of partial checksum field in
   transport header.

CRs-Fixed: 2156182
Change-Id: I49c01f0b3404d7897b3d2ab99f95ec69787e4b08
Signed-off-by: Subash Abhinov Kasiviswanathan <subashab@codeaurora.org>