wl12xx: Power off after flushing work

When stopping plt, the chip is powered off before all current work items
are flushed and interrupts are disabled. This might introduce a race in
which the driver tries to communicate with a powered off chip.

Fix this by powering off the device only after interrupts are disabled
and all work items are flushed.

Signed-off-by: Ido Yariv <ido@wizery.com>
Signed-off-by: Luciano Coelho <coelho@ti.com>