mxs/spi: Decrement the DMA/PIO border

This driver checks the length of transfer to be made and based
on this information, either chooses to transfer data via DMA or
PIO. Decrement this border further to gain better performace eg.
during SPI flash writes.

Empiric measurement shows that this gives extra 3kB/s write speed
with a M25P80 flash clocked at 40MHz.

Signed-off-by: Marek Vasut <marex@denx.de>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>