Revert "ALSA: usb-audio: Add quirk for Focusrite Scarlett

This reverts commit 1762a59d8e8b5e99f6f4a0f292b40f3cacb108ba.

This quirk is not needed because support for the Scarlett mixers will be added.

Signed-off-by: Chris J Arges <chris.j.arges@canonical.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
ALSA: usb-audio: Add private_data pointer to usb_mixer_elem_info

Add a private_data pointer to usb_mixer_elem_info to allow other mixer
implementations to extend the structure as necessary.

Signed-off-by: Chris J Arges <chris.j.arges@canonical.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
ALSA: usb-audio: make set_*_mix_values functions public

Make the functions set_cur_mix_value and get_cur_mix_value accessible by files
that include mixer.h. In addition make usb_mixer_elem_free accessible.
This allows reuse of these functions by mixers that may require quirks.

The following summarizes the renamed functions:
  - set_cur_mix_value -> snd_usb_set_cur_mix_value
  - get_cur_mix_value -> snd_usb_get_cur_mix_value
  - usb_mixer_elem_free -> snd_usb_mixer_elem_free

Signed-off-by: Chris J Arges <chris.j.arges@canonical.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
ALSA: usb-audio: Scarlett mixer interface for 6i6, 18i6, 18i8 and 18i20

This code contains the Scarlett mixer interface code that was originally
written by Tobias Hoffman and Robin Gareus. Because the device doesn't
properly implement UAC2 this code adds a mixer quirk for the device.

Changes from the original code include removing the metering code along with
dead code and comments. Compiler warnings were fixed. The code to initialize
the sampling rate was causing a crash this was fixed as discussed on the
mailing list. Error, and info messages were convered to dev_err and dev_info
interfaces. The custom scarlett_mixer_elem_info struct was replaced with the
more generic usb_mixer_elem_info to be able to recycle more code from mixer.c.

This patch also makes additional modifications based on upstream comments.
Individual control creation functions are removed and a generic
function is no used. Macros for function calls are removed to improve
readability. Hardcoded control initialization is removed. Save to HW
functionality has been removed. Strings for enums are created dynamically for
the mixer. Strings used for controls are now SNDRV_CTL_ELEM_ID_NAME_MAXLEN
length.

Signed-off-by: Chris J Arges <chris.j.arges@canonical.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>