backlight: tdo24m: ensure chip select changes between transfers

Some SPI host drivers do not change chip select betwen transfers unless
.cs_chnage field is explicitly set. The LCD spec requires chip select
change between consecuitive transfers, so ensure it at the SPI driver
level.

Signed-off-by: Mike Rapoport <mike@compulab.co.il>
Signed-off-by: Eric Miao <eric.y.miao@gmail.com>