vfio/pci: Fix OpRegion read

vfio/pci: Fix OpRegion read

This is to fix incorrect pointer arithmetic which caused wrong
OpRegion version returned, then VM driver got error to get wanted
VBT block. We need to be safe to return correct data, so force
pointer type for byte access.

Fixes: e695816fdda3 ("vfio/pci: Add OpRegion 2.0+ Extended VBT support.")
Cc: Colin Xu <colin.xu@gmail.com>
Cc: Alex Williamson <alex.williamson@redhat.com>
Cc: Dmitry Torokhov <dtor@chromium.org>
Cc: "Xu, Terrence" <terrence.xu@intel.com>
Cc: "Gao, Fred" <fred.gao@intel.com>
Acked-by: Colin Xu <colin.xu@gmail.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Link: https://lore.kernel.org/r/20211125051328.3359902-1-zhenyuw@linux.intel.com
[aw: line wrap]
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>