ALSA: memalloc: Allow NULL device for SNDRV_DMA_TYPE_CONTINUOUS type

ALSA: memalloc: Allow NULL device for SNDRV_DMA_TYPE_CONTINUOUS type

Currently we pass the artificial device pointer to the allocation
helper in the case of SNDRV_DMA_TYPE_CONTINUOUS for passing the GFP
flags.  But all common cases are the allocations with GFP_KERNEL, and
it's messy to put this in each place.

In this patch, the memalloc core helper is changed to accept the NULL
device pointer and it treats as the default mode, GFP_KERNEL, so that
all callers can omit the complex argument but just leave NULL.

Link: https://lore.kernel.org/r/20191105080138.1260-2-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>