ALSA: usb-audio: Use int for dB map values

The values in usbmix_dB_map should be rather signed while we're using
u32.  As the copied target (usb_mixer_elem_info.dBmin and dBmax) is
int, let's make them also int.

Link: https://lore.kernel.org/r/20211116065415.11159-2-tiwai@suse.de


Signed-off-by: Takashi Iwai <tiwai@suse.de>