Release RX descriptor after each frame

After each RX frame so HW can use it again. Otherwise RX buffer
will be full and HW stops sending frames to the host.

Signed-off-by: Eugene Krasnikov <k.eugene.e@gmail.com>
Rework RX routine to support both High and Low channels

Now RX routine is checking which channel triggered interrupt by
reading WCN36XX_DXE_INT_SRC_RAW_REG register and depends on that
process correct channel.

Signed-off-by: Eugene Krasnikov <k.eugene.e@gmail.com>
Fill in add_sta functionality

After association WCN expects to be configured
in POST-ASSOC state. Also configure new AID, STAID.

Signed-off-by: Eugene Krasnikov <k.eugene.e@gmail.com>