mac80211: fix scan race, simplify code

The scan code has a race that Michael reported
he ran into, but it's easy to fix while at the
same time simplifying the code.

The race resulted in the following warning:

------------[ cut here ]------------
WARNING: at net/mac80211/scan.c:310 ieee80211_rx_bss_free+0x20c/0x4b8 [mac80211]()
Modules linked in: [...]
[<c0033edc>] (unwind_backtrace+0x0/0xe0) from [<c004f2a4>] (warn_slowpath_common+0x4c/0x64)
[... backtrace wasn't useful ...]

Reported-by: Michael Buesch <mb@bu3sch.de>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>