ALSA: snd-usb: mixer: propagate errors up the call chain

In check_input_term() and parse_audio_feature_unit(), propagate the
error value that has been returned by a failing function. That helps
cleaning up the error pathes in the mixer.

Signed-off-by: Daniel Mack <zonque@gmail.com>
Reported-by: Rodolfo Thomazelli <pe.soberbo@gmail.com>
ALSA: snd-usb: mixer: ignore -EINVAL in snd_usb_mixer_controls()

Creation of individual mixer controls may fail, but that shouldn't cause
the entire mixer creation to fail. Even worse, such failures currently
cause the entire sound card creation to bail.

Signed-off-by: Daniel Mack <zonque@gmail.com>
Reported-by: Rodolfo Thomazelli <pe.soberbo@gmail.com>