dma_request_slave_channel() is a wrapper on top of dma_request_chan()
eating up the error code.

The dma_request_chan() is the standard API to request slave channel,
clients should be moved away from the legacy API to allow us to retire
them.
Acked-by: NViresh Kumar <viresh.kumar@linaro.org>
Acked-by: NBartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Signed-off-by: NPeter Ujfalusi <peter.ujfalusi@ti.com>
Signed-off-by: NJens Axboe <axboe@kernel.dk>