net: rmnet: set NETIF_F_LLTX flag

The rmnet_vnd_setup(), which is the callback of ->ndo_start_xmit() is
allowed to call concurrently because it uses RCU protected data.
So, it doesn't need tx lock.

Signed-off-by: Taehee Yoo <ap420073@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>