spi: pxa2xx: Use local struct spi_master pointer in pump_transfers()

We will find more use for struct spi_master pointer in pump_transfers()
and code will be more readable if we access it using local pointer than
through the drv_data->master.

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