ALSA: hda - rename dell_led_set_func to dell_micmute_led_set_func

With dell_app_wmi_led_set() replaced by dell_micmute_led_set(), rename
the function pointer to the latter for consistency.

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: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jacek Anaszewski <jacek.anaszewski@gmail.com>