spi/omap2_mcspi: change default DMA_MIN_BYTES value to 160

The value 160 has been obtained as optimal in testing it for
wl1271 which use spi for communication.
In some sense this change might also influence on other spi devices
connected to omap2_mcspi controller.

Signed-off-by: Roman Tereshonkov <roman.tereshonkov@nokia.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>