HID: lenovo: Map mic-mute button to KEY_F20 instead of KEY_MICMUTE

Mapping the mic-mute button to KEY_MICMUTE is technically correct but
KEY_MICMUTE translates to a scancode of 256 (248 + 8) under X,
which does not fit in 8 bits, so it does not work.

Because of this userspace is expecting KEY_F20 instead,
theoretically KEY_MICMUTE should work under Wayland but even
there it does not work, because the desktop-environment is
listening only for KEY_F20 and not for KEY_MICMUTE.

Fixes: bc04b37e ("HID: lenovo: Add ThinkPad 10 Ultrabook Keyboard support")
Reviewed-by: Marek Behún <kabel@kernel.org>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>