HID: elan: Silence mute LED errors being logged when the device is unplugged

HID: elan: Silence mute LED errors being logged when the device is unplugged

Being integrated into an USB keyboard-dock the mute LED can go away
at any time, leading to the following errors:

[  918.667671] elan 0003:04F3:0755.0002: Failed to set mute led brightness: -19
[  918.667737] leds elan:red:mute: Setting an LED's brightness failed (-19)

Fix this by making the following changes:

1. Don't log an error from elan_mute_led_set_brigtness() when
   ret == -ENODEV
2. Set the LED_HW_PLUGGABLE flag on the mute LED led_classdev

While at it also make sure that elan_mute_led_set_brigtness() returns
an error (-EIO) when ret != 3 but it is not an error (>= 0).

Reviewed-by: Marek Behún <kabel@kernel.org>
Acked-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>