spi: pxa2xx: Don't touch CS pin until we have a transfer pending

GPIO descriptors, when being requested, may configure pin at the same
time. In case of SPI chip select we shouldn't do any assumptions of the
state of pin since we don't know yet what chip is connected there and if
it uses high or low active state. So, leave the state of pin as is until
transfer will start.

Fixes: 99f499cd ("spi: pxa2xx: Add support for GPIO descriptor chip selects")
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Mika Westeberg <mika.westerberg@linux.intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>