spi: npcm-pspi: improve spi transfer performance

Improving spi 8 bit per word mode transfer performance
by using 16 bit per word transfer and receive when the data
length is even and larger than one.

Signed-off-by: Tomer Maimon <tmaimon77@gmail.com>
Link: https://lore.kernel.org/r/20200115162301.235926-3-tmaimon77@gmail.com


Signed-off-by: Mark Brown <broonie@kernel.org>