ALSA: usb-audio: constify snd_kcontrol_new structures

Declare snd_kcontrol_new structures as const as they are only passed as
an argument to the function add_new_ctl. This agrument is of type const,
so snd_kcontrol_new structures having this property can be made const
too.

Signed-off-by: Bhumika Goyal <bhumirks@gmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>