ALSA: usb-audio: fix missing input volume controls in MAYA44 USB(+)

The driver worked around an error in the MAYA44 USB(+)'s mixer unit
descriptor by aborting before parsing the missing field.  However,
aborting parsing too early prevented parsing of the other units
connected to this unit, so the capture mixer controls would be missing.

Fix this by moving the check for this descriptor error after the parsing
of the unit's input pins.

Reported-by: nightmixes <nightmixes@gmail.com>
Tested-by: nightmixes <nightmixes@gmail.com>
Signed-off-by: Clemens Ladisch <clemens@ladisch.de>
Cc: <stable@vger.kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>