[ALSA] Introduce snd_card_set_generic_dev()

ALSA Core
A new function snd_card_set_generic_dev() is introduced to add the
'generic device' support for devices without proper bus on sysfs.
It's a last resort, and should be removed in future when they have
a proper bus, instead.

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