spi: sh-msiof: Fix DMA transfer size check

DMA supports 32-bit words only,
even if BITLEN1 of SITMDR2 register is 16bit.

Fixes: b0d0ce8b ("spi: sh-msiof: Add DMA support")
Signed-off-by: Hiromitsu Yamasaki <hiromitsu.yamasaki.ym@renesas.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>
Acked-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Dirk Behme <dirk.behme@de.bosch.com>
Signed-off-by: Mark Brown <broonie@kernel.org>