p54: honour bss_info_changed's short slot time settings

This patch was made on behalf of Johannes request.
"mac80211 and IEEE80211_CONF_SHORT_SLOT_TIME"

Of course, bss_info_changed provides some more useful data.
e.g.: basic_rates, dtim_period, beacon_int and maybe even more.
Everything can be hooked up if it's necessary.

Signed-off-by: Christian Lamparter <chunkeey@web.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>