media: solo6x10: Remove superfluous snd_dma_continuous_data()

The recent change (commit 08422d2c: "ALSA: memalloc: Allow NULL
device for SNDRV_DMA_TYPE_CONTINOUS type") made the PCM preallocation
helper accepting NULL as the device pointer for the default usage.
Drop the snd_dma_continuous_data() usage that became superfluous from
the callers.

Acked-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Takashi Iwai <tiwai@suse.de>