cfg80211: allow users to request removing a BSS

This patch introduces cfg80211_unlink_bss, a function to
allow a driver to remove a BSS from the internal list and
make it not show up in scan results any more -- this is
to be used when the driver detects that the BSS is no
longer available.

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