spi: rockchip: configure CTRLR1 according to size and data frame

CTRLR1 is number of data frames, when rx only.
When data frame is 8 bit, CTRLR1 is len-1.
When data frame is 16 bit, CTRLR1 is (len/2)-1.

Signed-off-by: Huibin Hong <huibin.hong@rock-chips.com>
Signed-off-by: Mark Brown <broonie@kernel.org>