genirq/msi: Add missing kernel doc to msi_next_desc()

W=1 complains about this.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Ashok Raj <ashok.raj@intel.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Link: https://lore.kernel.org/r/20221111122013.947071142@linutronix.de