mac80211: fix slot time debug message

wlan0: switched to short barker preamble (BSSID=00:01:aa:bb:cc:dd)
wlan0: switched to short slot (BSSID=) <something is missing here>

should be:

wlan0: switched to short barker preamble (BSSID=00:01:aa:bb:cc:dd)
wlan0: switched to short slot (BSSID=00:01:aa:bb:cc:dd)

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