net: qualcomm: rmnet: Fix casting issues

Fix warnings which were reported when running with sparse
(make C=1 CF=-D__CHECK_ENDIAN__)

drivers/net/ethernet/qualcomm/rmnet/rmnet_handlers.c:81:15:
warning: cast to restricted __be16
drivers/net/ethernet/qualcomm/rmnet/rmnet_map_data.c:271:37:
warning: incorrect type in assignment (different base types)
expected unsigned short [unsigned] [usertype] pkt_len
got restricted __be16 [usertype] <noident>
drivers/net/ethernet/qualcomm/rmnet/rmnet_map_data.c:287:29:
warning: incorrect type in assignment (different base types)
expected unsigned short [unsigned] [usertype] pkt_len
got restricted __be16 [usertype] <noident>
drivers/net/ethernet/qualcomm/rmnet/rmnet_map_data.c:310:22:
warning: cast to restricted __be16
drivers/net/ethernet/qualcomm/rmnet/rmnet_map_data.c:319:13:
warning: cast to restricted __be16
drivers/net/ethernet/qualcomm/rmnet/rmnet_map_command.c:49:18:
warning: cast to restricted __be16
drivers/net/ethernet/qualcomm/rmnet/rmnet_map_command.c:50:18:
warning: cast to restricted __be32
drivers/net/ethernet/qualcomm/rmnet/rmnet_map_command.c:74:21:
warning: cast to restricted __be16

CRs-Fixed: 2156182
Change-Id: If037985ce26e63857125567b20a57f72e58e8d95
Signed-off-by: Subash Abhinov Kasiviswanathan <subashab@codeaurora.org>