wlcore: update channel_switch/stop_channel_switch commands

Some fields were added to the channel_switch and
stop_channel_switch commands. Unfortunately,
the new 18xx channel_switch struct is not backward
compatible with the 12xx channel switch struct.

Add a new channel_switch op to wlcore, and update
the driver accordingly.

Signed-off-by: Eliad Peller <eliad@wizery.com>
Signed-off-by: Luciano Coelho <coelho@ti.com>