ASoC: rsnd: set dmaen->chan = NULL when error case

rsnd_dmaen_quit() is assuming dmaen->chan is NULL if it failed
to get DMAEngine channel. but, current dmaen->chan might have
error value when error case (this driver is checking it by IS_ERR_OR_NULL())
This patch makes sure dmaen->chan is NULL when error case.
Otherwise, it will contact to unknown address in rsnd_dmaen_quit()

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Mark Brown <broonie@kernel.org>