ALSA: scarlett2: Add missing error check when initialise Autogain Status

ALSA: scarlett2: Add missing error check when initialise Autogain Status

When initialise new control with scarlett2_add_new_ctl() function for
Autogain Status, scarlett2_add_new_ctl() might throw an error. So, add
error check after initialise new control for Autogain Status.

This is reported by Coverity Scan with CID 1598781 as UNUSED_VALUE.

Fixes: 0a995e38dc44 ("ALSA: scarlett2: Add support for software-controllable input gain")
Signed-off-by: Robertus Diawan Chris <robertusdchris@gmail.com>
Link: https://patch.msgid.link/20260508033914.111596-1-robertusdchris@gmail.com
Signed-off-by: Takashi Iwai <tiwai@suse.de>