usb: usb251xb: Fix property_u32 NULL pointer dereference

The methods like of_property_read_u32 utilizing the specified
pointer permit only the pointer to a preallocated u32 storage as the
third argument. As a result the driver crashes on NULL pointer
dereference in case if "oc-delay-us" or "power-on-time-ms" declared
in dts file.

Signed-off-by: Serge Semin <fancer.lancer@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>