The description of object_property_get_int() stated that on an error
it returns NULL. This is not the case and the function will return -1
if an error occurs. Update the commented documentation accordingly.

Reported-By: Christian Liebhardt <christian.liebhardt@keysight.com>
Signed-off-by: Christian Liebhardt <christian.liebhardt@keysight.com>
Signed-off-by: Alistair Francis <alistair.francis@xilinx.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>