[ALSA] Add support for EDIROL UM-3ex

Modules: USB generic driver

This is my naive attempt at adding ALSA device support. The attached
patch provides support for the EDIROL UM-3ex. This is a 3-port USB midi
interface with a built-in USB hub and the ability to chain 2 other
UM-3x's in a master-slave configuration. I only have one, so I do not
know how this works in practice.

Though this is a 3-port device, I had to throw in that 4th 'Control' interface
to the definition in order to make the 3rd port work. If I set in/out_cables
to 0x000b, a 3rd interface appears on the driver, but it does nothing.
Changing it to 0x000f allows the 3rd interface to work, but of course
interface 4 does not work because it does not exist.

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