wl12xx: use a bitmask instead of list of booleans in scanned_ch

We were using an array of booleans to mark the channels we had already
scanned.  This was causing a sparse error, because bool is not a type
with defined size.  To fix this, use bitmasks instead, which is much
cleaner anyway.

Thanks Johannes Berg for the idea.

Signed-off-by: Luciano Coelho <coelho@ti.com>