platform/x86: dell-laptop: import dell_micmute_led_set() from drivers/leds/dell-led.c

To ensure all users of dell-smbios are in drivers/platform/x86, move the
dell_micmute_led_set() method from drivers/leds/dell-led.c to
drivers/platform/x86/dell-laptop.c.

Signed-off-by: Michał Kępień <kernel@kempniu.pl>
Tested-by: Alex Hung <alex.hung@canonical.com>
Reviewed-by: Pali Rohár <pali.rohar@gmail.com>
Acked-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Acked-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jacek Anaszewski <jacek.anaszewski@gmail.com>