ALSA: core: memalloc: fix fallthrough position

ALSA: core: memalloc: fix fallthrough position

Fix cppcheck, the fallthrough only makes sense within the conditional
block

sound/core/memalloc.c:161:3: style:inconclusive: Statements following
return, break, continue, goto or throw will never be
executed. [unreachableCode]
  fallthrough;
  ^

Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Link: https://lore.kernel.org/r/20200902212133.30964-3-pierre-louis.bossart@linux.intel.com
Signed-off-by: Takashi Iwai <tiwai@suse.de>