ALSA: usb-audio: Simplify parse_audio_unit()

Minor code refactoring by combining the UAC version and the type in
the switch-case flow, so that we reduce the indentation and
redundancy.  One good bonus is that the duplicated definition of the
same type value (e.g. UAC2_EFFECT_UNIT) can be handled more cleanly.

Signed-off-by: Takashi Iwai <tiwai@suse.de>