powerpc/pseries: Correct memory hotplug locking

Memory dlpar handling can return from dlpar_memory() without releasing the
device_hotplug lock. Correct this routine to ensure the lock is released.

Fixes: 5f97b2a0 ("powerpc/pseries: Implement memory hotplug add in the kernel")
Signed-off-by: Nathan Fontenot <nfont@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>