net: ethernet: rmnet: Always subtract MAP header

Commit e1d9a90a ("net: ethernet: rmnet: Support for ingress MAPv5
checksum offload") broke ingress handling for devices where
RMNET_FLAGS_INGRESS_MAP_CKSUMV5 or RMNET_FLAGS_INGRESS_MAP_CKSUMV4 are
not set. Unless either of these flags are set, the MAP header is not
removed. This commit restores the original logic by ensuring that the
MAP header is removed for all MAP packets.

Fixes: e1d9a90a

 ("net: ethernet: rmnet: Support for ingress MAPv5 checksum offload")
Signed-off-by: Kristian Evensen <kristian.evensen@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>