ALSA: usb-audio: Add input gain and master output mixer elements for RME Babyface Pro

ALSA: usb-audio: Add input gain and master output mixer elements for RME Babyface Pro

Add missing input gain and master output mixer controls for
RME Babyface Pro.

This patch implements:

1. Input gain controls for 2 mic and 2 line inputs
2. Master output volume controls for all 12 output channels

These additions allow for more complete control of the Babyface Pro under
Linux.

Signed-off-by: Stefan Stistrup <sstistrup@gmail.com>
Link: https://patch.msgid.link/20240809204922.20112-1-sstistrup@gmail.com
Signed-off-by: Takashi Iwai <tiwai@suse.de>