wl12xx: Add support for HW channel switch

The wl12xx FW supports HW channel switch.  If we don't use it,
sometimes the firmware gets confused when recalibrating to the new
channel, causing RX problems.  This commit adds HW channel switch
support by implementing the channell_switch op.

Signed-off-by: Shahar Levi <shahar_levi@ti.com>
[added one comment, remove the tx_flush and rephrased the commit message]
Signed-off-by: Luciano Coelho <coelho@ti.com>