brcmfmac: fix a double spin_unlock_irqrestore issue in dpc

dpc_tl_lock is not acquired in the error handle code for bus down.
But it's unlocked using spin_unlock_irqrestore after finishing task
list walk down. Grab the lock before breaking the loop to avoid a
double unlock.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Franky Lin <frankyl@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>