Staging: w35und: Fix ->beacon_int breakage

Commit f424afa1 ("mac80211: remove
deprecated API") removed ->beacon_int from struct ieee80211_conf. Fix
breakage in w35und by setting beacon period in ->add_interface to
bss_conf.beacon_int.

Cc: Jiri Benc <jbenc@suse.cz>
Cc: Jiri Kosina <jkosina@suse.cz>
Cc: Johannes Berg <johannes@sipsolutions.net>
Cc: Pavel Machek <pavel@ucw.cz>
Cc: Sandro Bonazzola <sandro.bonazzola@gmail.com>
Signed-off-by: Pekka Enberg <penberg@cs.helsinki.fi>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>