backlight: ili922x: Remove redundant variable len

The variable len is assigned but never read, therefore it is redundant
and can be removed. Cleans up clang warning:

drivers/video/backlight/ili922x.c:276:2: warning: Value stored to 'len'
is never read

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Acked-by: Daniel Thompson <daniel.thompson@linaro.org>
Acked-by: Jingoo Han <jingoohan1@gmail.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>