ALSA: Add ifdef CONFIG_GENERIC_ALLOCATOR for SNDRV_DMA_TYPE_IRAM code

It turned out that we can't use gen_pool_*() functions on archs
without CONFIG_GENERIC_ALLOCATOR (resulting in missing symbols), since
linux/genalloc.h doesn't provide dummy functions for all.  We'd be
able to fix linux/genalloc.h size, but I take an easier path for
now...

Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>