PCI: add PCI-X/PCI-Express read control interfaces

This patch introduces an interface to read and write PCI-X / PCI-Express 
maximum read byte count values from PCI config space. There is a second 
function that returns the maximum _designed_ read byte count, which marks the 
maximum value for a device, since some drivers try to set MMRBC to the 
highest allowed value and rely on such a function.

Based on patch set by Stephen Hemminger <shemminger@linux-foundation.org>

Cc: Stephen Hemminger <shemminger@linux-foundation.org>
Signed-off-by: Peter Oruba <peter.oruba@amd.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>