backlight: lm3630a: Return 0 on success in update_status functions

lm3630a_bank_a_update_status() and lm3630a_bank_b_update_status()
both return the brightness value if the brightness was successfully
updated. Writing to these attributes via sysfs would cause a 'Bad
address' error to be returned. These functions should return 0 on
success, so let's change it to correct that error.

Fixes: 28e64a68 ("backlight: lm3630: apply chip revision")
Signed-off-by: Brian Masney <masneyb@onstation.org>
Acked-by: Pavel Machek <pavel@ucw.cz>
Acked-by: Daniel Thompson <daniel.thompson@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>