leds: rgb: leds-mt6370-rgb: Improve definition of some struct linear_range

Use LINEAR_RANGE() instead of hand-writing it. It is more robust, should
the layout of the structure change one day.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Link: https://lore.kernel.org/r/1ce4245107e0a51dce502a007a69899bda018d5f.1746197460.git.christophe.jaillet@wanadoo.fr


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