cxl: Test the correct mmio space before unmapping

Before freeing p2n, test p2n, not p1n.

Signed-off-by: Daniel Axtens <dja@axtens.net>
Acked-by: Ian Munsie <imunsie@au1.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>