[ALSA] s3c24xx-pcm: fix hw_params dma handling

Since the PCM emulation can call multiple times to hw_setup(), but we
can only once allocate/request the DMA channel, we have to handle
this gracefully.

Signed-off-by: Harald Welte <laforge@openmoko.org>
Signed-off-by: Arnaud Patard <arnaud.patard@rtp-net.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@suse.cz>