DFS: Handle pre-CAC expired event

DFS: Handle pre-CAC expired event

As FCC DFS requirement does not explicitly mention about the validity of
the (pre-)CAC when channel is switched, it is safe to assume that the
pre-CAC result will not be valid once the CAC completed channel is
switched or radar detection is not active on the (CAC completed) channel
within a time period which is allowed (10 seconds - channel switch time)
as per FCC DFS requirement.

Use the new driver event to allow the driver to notify expiry of the CAC
result on a channel. Move the DFS state of the channel to 'usable' when
processing pre-CAC expired event.  This means any future operation on
that channel will require a new CAC to be completed. This event is
applicable only when DFS is not offloaded to the kernel driver.

Signed-off-by: Vasanthakumar Thiagarajan <vthiagar@qti.qualcomm.com>