zd1211rw: remove false assertion from zd_mac_clear()

The function is called before the lock which is asserted was ever used.
Just remove it.

Reported-by:  <syzbot+74c65761783d66a9c97c@syzkaller.appspotmail.com>
Signed-off-by: Oliver Neukum <oneukum@suse.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>