dma: mv_xor: fix error checking of irq_of_parse_and_map()

The irq_of_parse_and_map() function returns 0 on failure, and does not
return an error code, so we fix the calling site of
irq_of_parse_and_map() in the mv_xor driver.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>