ALSA: memalloc: Make SG-buffer helper usable for continuous buffer, too

We have a few helper functions for making the access to the buffer
address easier on SG-buffer.  Those are specific to the buffer that is
allocated with SG-buffer type, and it makes hard to use both SG and
non-SG buffers in the same code.

This patch adds a few simple checks and lets the helpers to deal with
both SG- and continuous buffers gracefully.  It's a preliminary step
for the upcoming patch that mimics the buffer type on the fly.

Link: https://lore.kernel.org/r/20200615160045.2703-4-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>