[POWERPC] Inline u3msi_compose_msi_msg()

In the MPIC U3 MSI code, we call u3msi_compose_msi_msg() once for each MSI.
This is overkill, as the address is per pci device, not per MSI. So setup
the address once, and just set the data per MSI.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>