dma: mv_xor: fix error handling of mv_xor_channel_add()

When mv_xor_channel_add() fails for one XOR channel, we jump to the
err_channel_add label to clean up all previous channels that had been
initialized correctly. Unfortunately, while handling this error
condition, we were disposing the IRQ mapping before calling
mv_xor_channel_remove() (which does the free_irq()), which is
incorrect.

Instead, do things properly in the reverse order of the
initialization: first remove the XOR channel (so that free_irq() is
done), and then dispose the IRQ mapping.

This avoids ugly warnings when for some reason one of the XOR channel
fails to initialize.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>