dma: mv_xor: use request_irq() instead of devm_request_irq()

Even through the usage of devm_*() functions is generally recommended
over their classic variants, in the case of devm_request_irq()
combined with irq_of_parse_and_map(), it doesn't work nicely.

We have the following scenario:

 irq_of_parse_and_map(...)
 devm_request_irq(...)

For some reason, the driver initialization fails at a later
point. Since irq_of_parse_and_map() is no device-managed, we do a:

 irq_dispose_mapping(...)

Unfortunately, this doesn't work, because the free_irq() must be done
prior to calling irq_dispose_mapping(). But with the devm mechanism,
the automatic free_irq() would happen only after we get out of the
->probe() function.

So basically, we revert to using request_irq() with traditional error
handling, so that in case of error, free_irq() gets called before
irq_dispose_mapping().

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