powerpc/pseries: Fix creation of loop in device node property list

The update_dt_prop helper function fails to set the IN/OUT parameter prop to
NULL after a complete property has been parsed from the work area returned by
the ibm,update-properties rtas function. This results in the property list of
the device node being updated is corrupted and becomes a loop since the same
property structure is used repeatedly.

Signed-off-by: Tyrel Datwyler <tyreld@linux.vnet.ibm.com>
Acked-by: Nathan Fontenot <nfont@linux.vnet.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>