serial: atmel: convert not to use dma_request_slave_channel()

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/f2e9790d8b49aeba8b43ce018d30a35b837ac1eb.1700409299.git.christophe.jaillet@wanadoo.fr


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>