dma: mv_xor: fix error handling for clocks

When a channel fails to initialize, we release all ressources,
including clocks. However, a XOR unit is not necessarily associated to
a clock (some variants of Marvell SoCs have a clock for XOR units,
some don't), so we shouldn't unconditionally be releasing the clock.

Instead, just like we do in the mv_xor_remove() function, we should
check if one clock was found before releasing it.

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