ALSA: oxfw: change function prototype for AV/C Audio Subunit command

ALSA OXFW driver uses AV/C Audio Subunit commands to control some models.
The commands get/set the state of Feature function block of the subunit.
The commands are not specific to OXFW, thus there's a possibility to use
them in the other drivers.

Currently, helper functions for the commands require 'struct snd_oxfw',
although, it's not necessarily required. It's better to change prototype
of the functions without the structure for future use.

This commit changes the prototype.

Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: Takashi Iwai <tiwai@suse.de>