usb: gadget: uac2: provide a variable for interface and alt settings

This patch removes the shifting and masking of interface and its alt
setting and provides its own variable.

This looks better and is smaller:
 text    data     bss     dec     hex filename
x86-32
 6940     956      56    7952    1f10 gadget/audio.o.old
 6908     956      56    7920    1ef0 gadget/audio.o.new
arm
 7914     956      56    8926    22de gadget/audio.o.old
 7886     956      56    8898    22c2 gadget/audio.o.new

Acked-by: Jassi Brar <jassisinghbrar@gmail.com>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Felipe Balbi <balbi@ti.com>