net: qualcomm: rmnet: Move common struct definitions to include

Create if_rmnet.h and move the rmnet MAP packet structs to this
common include file. To account for portablity, add little and
big endian bitfield definitions similar to the ip & tcp headers.

The definitions in the headers can now be re-used by the
upcoming ipa driver series as well as qmi_wwan.

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