ASoC: sh: rz-ssi: Improve error handling in rz_ssi_dma_request function

ASoC: sh: rz-ssi: Improve error handling in rz_ssi_dma_request function

dma_request_chan() returns error pointer in case of failures, but
the rz_ssi_dma_request() checked for NULL pointer instead.

This patch fixes the issue by checking for ERR_PTR() instead of
NULL and sets the DMA pointers to NULL in error case so that ssi
can fallback to PIO mode.

Fixes: 26ac471c5354 ("ASoC: sh: rz-ssi: Add SSI DMAC support")
Signed-off-by: Biju Das <biju.das.jz@bp.renesas.com>
Reviewed-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Link: https://lore.kernel.org/r/20210818101450.15948-1-biju.das.jz@bp.renesas.com
Signed-off-by: Mark Brown <broonie@kernel.org>