genirq/msi: Clean up usage of __u8/__u16 types

The double underscore types are meant for compatibility in userspace
headers which does not apply here. Therefore, change to use the standard
no-underscore types.

The origin of the double underscore types dates back to before the git era
so I was not able to find a commit to see the original justification.

Signed-off-by: Logan Gunthorpe <logang@deltatee.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>