wl1271: rewritten scanning code

This patch is a complete rewrite of the scanning code.  It now includes a
state machine to scan all four possible sets of channels independently:
2.4GHz active, 2.4GHz passive, 5GHz active and 5GHz passive.  The wl1271
firmware doesn't allow these sets to be mixed, so up to several scan commands
have to be issued.  This patch also allows scanning more than 24 channels per
set, by breaking the sets into smaller parts if needed (the firmware can scan
a maximum of 24 channels at a time).

Previously, the scanning code was erroneously scanning all channels possible
actively, not complying with the CRDA values.  This is also fixed with this
patch.

Signed-off-by: Luciano Coelho <luciano.coelho@nokia.com>
Reviewed-by: Saravanan Dhanabal <ext-saravanan.dhanabal@nokia.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>