acer-wmi: ignore missing Aspire 5741G keys

Ignore Aspire's 5741G:
 KEY_PREVIOUSSONG
 KEY_NEXTSONG
 KEY_PLAYPAUSE
 KEY_STOP
 KEY_VOLUMEDOWN
Signed-off-by: NSergey Senozhatsky <sergey.senozhatsky@gmail.com>
Signed-off-by: NMatthew Garrett <mjg@redhat.com>