mac80211: invoke set_tim() callback after setting own TIM info

Drivers should be allowed to simply get a complete new beacon when
set_tim() is invoked (and set_tim() is required for drivers that
just want a beacon template!), so we need to update our own TIM
bitmap before calling set_tim() so that getting the beacon will
now get an already updated beacon.

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