backlight: lm3630a: Don't set bl->props.brightness in get_brightness

There's no need to set bl->props.brightness, the get_brightness function
is just supposed to return the current brightness and not touch the
struct.

With that done we can also remove the 'goto out' and just return the
value.

Fixes: 0c2a665a

 ("backlight: add Backlight driver for lm3630 chip")
Signed-off-by: Luca Weiss <luca@z3ntu.xyz>
Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>
Link: https://lore.kernel.org/r/20240220-lm3630a-fixups-v1-2-9ca62f7e4a33@z3ntu.xyz
Signed-off-by: Lee Jones <lee@kernel.org>