ALSA: usb/mixer: remove bogus cast

"uinfo->value.enumerated.item" is an unsigned int.  If it's negative
when we do the comparison:
	if ((int)uinfo->value.enumerated.item >= cval->max)
then we would read past the end of the array on the next line.

I also changed the strcpy() to strlcpy() out of paranoia.

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