edac: mpc85xx use resource_size instead of raw math

Use resource_size() instead of arithmetic.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Doug Thompson <dougthompson@xmission.com>
Acked-by: Dave Jiang <djiang@mvista.com>
Cc: Peter Tyser <ptyser@xes-inc.com>
Cc: Kumar Gala <galak@gate.crashing.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>