[SCSI] sr: split sr_audio_ioctl into specific helpers

split each ioctl handled in sr_audio_ioctl into a function of it's own.
This cleans the code up nicely, and allows various places in sr_ioctl
to call these helpers directly instead of going through the multiplexer.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Jens Axboe <axboe@suse.de>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>