[ARM] 5122/1: imx_dma_request_by_prio simpilfication

imx_dma_request_by_prio can return channel number by itself.
No need to supply variable address through parameters.

Also converted all drivers using this function.

Signed-off-by: Paulius Zaleckas <paulius.zaleckas@teltonika.lt>
Acked-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>