spi/pxa2xx: change default supported DMA burst size to 1

This is to fix the SPI DMA transfer failure for speed less than 1M.
If using current DMA burst size setting (16), the Rx data bytes are
invalid due to each data byte is multiplied according to the burst
size setting.

Let's said supposedly we shall receive the following 18 bytes of data:
01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18
Instead, the data bytes received consist of "16 bytes of '01' +
2 bytes of '02'" :
01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 02 02

Signed-off-by: Chew, Chiau Ee <chiau.ee.chew@intel.com>
Acked-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Mark Brown <broonie@linaro.org>