mac80211: remove IEEE80211_HW_SCAN_WHILE_IDLE

There are only a few drivers that use HW scan, and
all of those don't need a non-idle transition before
starting the scan -- some don't even care about idle
at all. Remove the flag and code associated with it.

The only driver that really actually needed this is
wl1251 and it can just do it itself in the hw_scan
callback -- implement that.

Acked-by: Luciano Coelho <coelho@ti.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>