staging: wfx: wfx_flush() did not ensure that frames are processed

staging: wfx: wfx_flush() did not ensure that frames are processed

wfx_flush() exited once all frames are retrieved from the device.
However, it did not ensure they were processed by driver before to
return. Therefore, some frame may be processed after the interface has
disappear.

Change the place we signal that the queue is empty to fix that.

Reviewed-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Jérôme Pouiller <jerome.pouiller@silabs.com>
Link: https://lore.kernel.org/r/20200401110405.80282-30-Jerome.Pouiller@silabs.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>