platform/x86: dell-laptop: don't register micmute LED if there is no token

On Dell G3-3590, error message is issued during boot up,
"platform::micmute: Setting an LED's brightness failed (-19)",
but there's no micmute led on the machine.

Get the related tokens of SMBIOS, GLOBAL_MIC_MUTE_DISABLE/ENABLE.
If one of two tokens doesn't exist,
don't call led_classdev_register() for platform::micmute.
After that, you wouldn't see the platform::micmute in /sys/class/leds/,
and the error message wouldn't see in dmesg.

Fixes: d00fa46e

 ("platform/x86: dell-laptop: Add micmute LED trigger support")
Signed-off-by: Koba Ko <koba.ko@canonical.com>
Reviewed-by: Mario Limonciello <Mario.limonciello@dell.com>
Reviewed-by: Pali Rohár <pali@kernel.org>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>