net: rmnet_data: Add RXCSUM capability to netdevices

Checksum offload is supported in MAPv3/v4. Add the device feature
to indicate this support.

Change-Id: I89caf6d9029cd483759404542681621909de70a3
Signed-off-by: Subash Abhinov Kasiviswanathan <subashab@codeaurora.org>