ALSA: control: add support for ENUMERATED user space controls

Handling of user control elements was implemented for all types except
ENUMERATED.  This type will be needed for the device-specific mixers of
upcoming FireWire drivers.

Signed-off-by: Clemens Ladisch <clemens@ladisch.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>