leds: core: Refactor led_update_brightness() to use standard pattern

leds: core: Refactor led_update_brightness() to use standard pattern

The standard conditional pattern is to check for errors first and
bail out if any. Refactor led_update_brightness() accordingly.

While at it, drop unneeded assignment and return 0 unconditionally
on success.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Denis Osterland-Heim <denis.osterland@diehl.com>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Link: https://lore.kernel.org/r/20231016153051.1409074-1-andriy.shevchenko@linux.intel.com
Signed-off-by: Lee Jones <lee@kernel.org>