spi: pxa2xx: Use dummy buffers provided by SPI core

Dummy buffer is used for half duplex transfers that don't have TX or RX
buffer set. Instead of own dummy buffer management here let the SPI core to
handle it by setting the SPI_MASTER_MUST_RX and SPI_MASTER_MUST_TX flags.
Then core makes sure both transfer buffers are set.

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