mac80211: rework scanning to account for probe response/beacon difference

This patch reworks the scanning code (ieee80211_rx_bss_info) to take
more parameters from beacons and keep a BSS info structure alive when
only beacons for it are received. This fixes a problem with iwlwifi
drivers (where we don't understand the root cause of the problem yet)
and another driver for some broken hardware (which cannot send probe
requests unless associated, so can't always actively scan.)

Signed-off-by: Bill Moss <bmoss@clemson.edu>
[jmberg: reformatted comments, make probe_resp a bool]
Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>