dmaengine: mv_xor_v2: Get rid of msi_desc abuse

Storing a pointer to the MSI descriptor just to keep track of the Linux
interrupt number is daft. Use msi_get_virq() instead.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Acked-by: Vinod Koul <vkoul@kernel.org>
Link: https://lore.kernel.org/r/20211210221814.970099984@linutronix.de