ALSA: oxfw: fix detect_loud_models() return value

This code causes a static checker warning:

	sound/firewire/oxfw/oxfw.c:46 detect_loud_models()
	warn: signedness bug returning '(-2)'

The detect_loud_models() function should return false on falure, so that
we don't try to set up the loud code for hardware that doesn't support
it.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: Takashi Iwai <tiwai@suse.de>