m32r: Fix set_memory() for DISCONTIGMEM

In case CONFIG_DISCONTIGMEM is set, the memory size of system was
always determined by CONFIG_MEMORY_SIZE and was not changeable.

This patch fixes set_memory() of arch/m32r/mm/discontig.c so that
we can specify memory size by the "mem=<size>" kernel parameter.

Signed-off-by: Hirokazu Takata <takata@linux-m32r.org>