spi: pxa2xx: Use core message processing loop

Convert the pump_transfers() transfer tasklet to transfer_one() hook the
SPI core calls to process single transfer instead of handling message
processing and chip select handling in the driver. This not only
simplifies the driver but also brings transfer statistics from the core.

Signed-off-by: Jarkko Nikula <jarkko.nikula@linux.intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>