usb: gadget: u_audio: Remove superfluous snd_dma_continuous_data()

The recent change (commit 08422d2c: "ALSA: memalloc: Allow NULL
device for SNDRV_DMA_TYPE_CONTINUOUS 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: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Link: https://lore.kernel.org/r/20191108164214.611-1-tiwai@suse.de


Signed-off-by: Takashi Iwai <tiwai@suse.de>