ALSA: scarlett2: Remove struct scarlett2_usb_volume_status

ALSA: scarlett2: Remove struct scarlett2_usb_volume_status

The struct scarlett2_usb_volume_status matched the config space layout
of a few volume controls that could be read together and were in fixed
locations between Gen 2 and Gen 3 devices.

Gen 4 devices have removed, moved, and new related controls, so this
needs to be cleaned up. By adding SCARLETT2_CONFIG_MASTER_VOLUME (the
only config item that didn't already have its own entry, because it is
read-only), we can remove:
- struct scarlett2_usb_volume_state,
- #define SCARLETT2_USB_VOLUME_STATUS_OFFSET, and
- scarlett2_usb_get_volume_status()
and replace with calls to scarlett2_usb_get_config().

Signed-off-by: Geoffrey D. Bennett <g@b4.vu>
Link: https://lore.kernel.org/r/2ee88994857246bf89fab8e62ac279f3bcf96192.1703444932.git.g@b4.vu
Signed-off-by: Takashi Iwai <tiwai@suse.de>