spi: pxa2xx: Fix cs_change management

Fix cs_change management so that it is in line with other spi drivers.

In the spi core api helpers such as spi_bus_lock/unlock and spi_sync_locked
or cs_change field in spi_transfer help to manage chip select from the
device driver.

The driver was setting the chip select to idle if the message queue was
empty despite cs_change or other status field set by spi_bus_lock/unlock
or spi_sync_locked.

Signed-off-by: Christophe Ricard <christophe-h.ricard@st.com>
Signed-off-by: Mark Brown <broonie@kernel.org>