backlight: ili922x: Add an error code check in ili922x_write()

Clang static analyzer complains that value stored to 'ret' is never read.
Return the error code when spi_sync() failed.

Signed-off-by: Su Hui <suhui@nfschina.com>
Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>
Link: https://lore.kernel.org/r/20231130051155.1235972-1-suhui@nfschina.com


Signed-off-by: Lee Jones <lee@kernel.org>