mailbox: bcm-flexrm-mailbox: Rework MSI interrupt handling

No point in retrieving the MSI descriptors. Just query the Linux interrupt
number.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Link: https://lore.kernel.org/r/20211210221815.148331680@linutronix.de