memory: Simplify mr_add_subregion() if-else

This if else is not needed. The previous call to memory_region_add
(whether _overlap or not) will always set priority and may_overlap
to desired values. And its not possible to get here without having
called memory_region_add_subregion due to the null guard on parent.
So we can just directly call memory_region_add_subregion_common.

Signed-off-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
