dma_request_slave_channel() is deprecated. dma_request_chan() should
be used directly instead.

Switch to the preferred function and update the error handling accordingly.
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Link: https://lore.kernel.org/r/1c88236b5d6bff0af902492ea9e066c8cb0dfef5.1700391566.git.christophe.jaillet@wanadoo.frSigned-off-by: Mark Brown <broonie@kernel.org>