ALSA: Make snd_sgbuf_get_{ptr|addr}() available for non-SG cases

Passing struct snd_dma_buffer pointer instead, so that they work no
matter whether real SG buffer is used or not.

This is a preliminary work for the HD-audio DSP loader code.

Signed-off-by: Ian Minett <ian_minett@creativelabs.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>