iwlegacy: Add a lock assertion in il4965_send_rxon_assoc()

The variables il->staging.filter_flags, rxon1->filter_flags and
rxon2->filter_flags need to be protected by the mutex lock il->mutex.
This patch adds a lock assertion of il->mutex to check whether
this lock is held.

Signed-off-by: Jia-Ju Bai <baijiaju1990@gmail.com>
Acked-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>