Fix MB calculation in the reporting of the MTRR hole

Change-Id: I34b5c4ffd2a3f3e895d2bffedce1c00ee9aea942
Signed-off-by: Duncan Laurie <dlaurie@google.com>
Reviewed-on: http://review.coreboot.org/763
Tested-by: build bot (Jenkins)
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>