mac80211: fix probe response wait timing

In "mac80211: split out and decrease probe wait time" I tried
to reduce the time waiting for a probe response, but failed to
take into account the case where we are detecting beacon loss
in software -- in that case we still wait the monitoring time
rather than the probe wait time. Fix this by refactoring the
mod_timer() calls in ieee80211_associated().

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