ALSA: usb-audio: Don't start stream for capture at prepare

The recent change made mistakenly the stream for capture started at
prepare stage.  Add the stream direction check to avoid it.

Fixes: 9c9a3b9d ("ALSA: usb-audio: Rename early_playback_start flag with lowlatency_playback")
Link: https://lore.kernel.org/r/20211119102629.7476-1-tiwai@suse.de


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