wlcore: Prevent processing of work items during op_stop

The interrupt line is disabled in op_stop using disable_irq. Since
pending interrupts are synchronized, the mutex has to be released before
disabling the interrupt to avoid a deadlock with the interrupt handler.

In addition, the internal state of the driver is only set to 'off'
after the interrupt is disabled. Otherwise, if an interrupt fires after
the state is set but before the interrupt line is disabled, the
interrupt handler will not be able to acknowledge the interrupt
resulting in an interrupt storm.

The driver's operations might be called during recovery. If these
acquire the mutex after it was released by op_stop, but before the
driver's state is changed, they may queue new work items instead of just
failing. This is especially problematic in the case of scans, in which a
new scan may be scheduled after all scan requests were cancelled.

Signed-off-by: Ido Yariv <ido@wizery.com>
Signed-off-by: Arik Nemtsov <arik@wizery.com>