Instead of calling each time device_create_file(), create the groups
of sysfs attribute files at once in a normal way.  Add a new helper
function, snd_get_device(), to return the associated device object,
so that we can handle the sysfs addition locally.

Since the sysfs file addition is done differently now,
snd_add_device_sysfs_file() helper function is removed.
Signed-off-by: Takashi Iwai <tiwai@suse.de>