mac80211: fix association beacon wait timeout

The TU_TO_EXP_TIME() macro already includes the
"jiffies +" piece of the calculation, so don't
add jiffies again.

Reported-by: Oliver Hartkopp <socketcan@hartkopp.net>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Tested-by: Oliver Hartkopp <socketcan@hartkopp.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>