Fix missing cast back to void *

MEM_RANGE_PTR() also needs to return a pointer to untyped memory.

Change-Id: I0ec64ad7bdb136d5e1a999bff3df6fa66eb29bf1
Signed-off-by: Philip Prindeville <philipp@redfish-solutions.com>
Reviewed-on: http://review.coreboot.org/500
Tested-by: build bot (Jenkins)
Reviewed-by: Patrick Georgi <patrick@georgi-clan.de>