usb: cdns3: fix NULL pointer dereference on no platform data

Some platforms (e.g. TI) will not have any platform data which will
lead to NULL pointer dereference if we don't check for NULL pdata.

Fixes: 7cea9657 ("usb: cdns3: add quirk for enable runtime pm by default")
Reported-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Roger Quadros <rogerq@ti.com>
Acked-by: Pawel Laszczak <pawell@cadence.com>
Signed-off-by: Peter Chen <peter.chen@nxp.com>