spapr_drc: use g_strdup_printf() instead of snprintf()

Passing a stack allocated buffer of arbitrary length to snprintf()
without checking the return value can cause the resultant strings
to be silently truncated.

Signed-off-by: Greg Kurz <groug@kaod.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
