platform/x86: asus-nb-wmi: Add microphone mute key code

The microphone mute key is missing from sparse keymap. It is present on
FX505GM and possibly other laptops. Add the missing code.

Also, comment on the fan mode switch key, which has the same code as the
already used key.

Signed-off-by: Yurii Pavlovskyi <yurii.pavlovskyi@gmail.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>