[PATCH] powerpc/fsl_msi: clean up and document calculation of MSIIR address

[Tabi Timur-B04825]

Timur Tabi wrote:
> Commit 3da34aae (powerpc/fsl: Support unique MSI addresses per PCIe Root
> Complex) redefined the meanings of msi->msi_addr_hi and msi->msi_addr_lo to be
> an offset rather than an address.  To help clarify the code, we make the
> following changes:

Kumar, I'm going to merge this patch into a bigger patch that makes a 
bunch of other fixes.