mac80211: refactor and move scan RX code

This patch refactors some code and moves the scan RX function
to scan.c. More importantly, however, it changes it so that the
MLME's beacon/probe_resp functions aren't invoked when scanning
so that we can remove a "if (scanning)" conditions from two
places.

There's a very slight behavioural change in this patch: now,
when scanning, IBSS and mesh aren't updated even on the same
channel.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>