spi: Unlock a spinlock before calling into the controller driver.

spi_pump_messages() calls into a controller driver with
unprepare_transfer_hardware() which is documented as "This may sleep".
As in the prepare_transfer_hardware() call below, we should release the
queue_lock spinlock before making the call.
Rework the logic a bit to hold queue_lock to protect the 'busy' flag,
then release it to call unprepare_transfer_hardware().

Signed-off-by: Bryan Freed <bfreed@chromium.org>
Reviewed-by: Doug Anderson <dianders@chromium.org>
Signed-off-by: Doug Anderson <dianders@chromium.org>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>