MSI: Use correct data offset for 32-bit MSI in read_msi_msg()

While reading the MSI code trying to find a reason why MSI wouldn't
work for devices that have a 32-bit MSI address capability, I noticed
that read_msi_msg() seems to read the message data from the wrong
offset in this case.

Signed-off-by: Roland Dreier <roland@digitalvampire.org>
Acked-by: Eric W. Biederman <ebiederm@xmission.com>
Cc: stable <stable@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>