[ALSA] dynamic minors (1/6): store device type in struct snd_minor

Instead of a comment string, store the device type in the snd_minor
structure.  This makes snd_minor more flexible, and has the nice side
effect that we don't need anymore to create a separate snd_minor
template for registering a device but can pass the file_operations
directly to snd_register_device().

Signed-off-by: Clemens Ladisch <clemens@ladisch.de>