Staging: w35und: unregister device from the ieee80211 stack upon ->disconnect()

Impact: fix module removal

This patch fixes an oops when the w35und module is removed from the
kernel and added back.

Reported-by: luoyi <luoyi.ly@gmail.com>
Tested-by: Sandro Bonazzola <sandro.bonazzola@gmail.com>
Cc: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Pekka Enberg <penberg@cs.helsinki.fi>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>