ALSA: USB: 6fire: signedness bug in usb6fire_pcm_prepare()

rt->rate is an unsigned char so it's never equal to -1.  It's not a huge
problem because the invalid rate is caught inside the call to
usb6fire_pcm_set_rate() which returns -EINVAL.  But if we fix the test
then it prints out the correct error message so that's good.

Signed-off-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>