ACPI: property: Polish ignoring bad data nodes

Clean up ignoring data nodes in mipi-disco-img.c: use { } initialiser, fix
a kernel-doc warning and use isdigit().

Fixes: 5bd4edbb ("ACPI: property: Ignore bad graph port nodes on Dell XPS 9315")
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>