[POWERPC] Fix mem= handling when the memory limit is > RMO size

There's a bug in my cleaned up mem= handling, if the memory limit is
larger than the RMO size we'll erroneously enlarge the RMO size.

Fix is to only change the RMO size if the memory limit is less than
the current RMO value.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>