media: v4l: fwnode: Fix lane-polarities property parsing

fwnode_property_read_u32_array() only returns the number of array elements
if the array argument is NULL. The assumption that it always did so lead to
lane-polarities properties never being read.

Fixes: 4ee23621 ("media: v4l2-fwnode: suppress a warning at OF parsing logic")

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>