ALSA: pcm: Fix build error on m68k and others

The commit 3ad796cb ("ALSA: pcm: Use SG-buffer only when direct
DMA is available") introduced a check of the DMA type and this caused
a build error on m68k (and possibly some others) due to the lack of
dma_is_direct() definition.  Since the check is needed only for
CONFIG_SND_DMA_SGBUF enablement (i.e. solely x86), use #ifdef instead
of IS_ENABLED() for avoiding such a build error.

Fixes: 3ad796cb

 ("ALSA: pcm: Use SG-buffer only when direct DMA is available")
Reported-by: kernel test robot <lkp@intel.com>
Link: https://lore.kernel.org/r/20200707111225.26826-1-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>