leds: led-class: Add led_module_get() helper

Split out part of of_led_get() into a generic led_module_get() helper
function.

This is a preparation patch for adding a generic (non devicetree specific)
led_get() function.

Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Lee Jones <lee@kernel.org>
Link: https://lore.kernel.org/r/20230120114524.408368-3-hdegoede@redhat.com