iommu/amd: Fix union of bitfields in intcapxt support

All the bitfields in here are overlaid on top of each other since
they're a union. Change the second u64 to be in a struct so it does
the intended thing.

Fixes: b5c3786e ("iommu/amd: Use msi_msg shadow structs")
Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20201111144322.1659970-2-dwmw2@infradead.org