usb: dwc3: core: cope with NULL pdata

if pdata is a NULL pointer we could cause a
kernel oops when probing the driver. Make sure
to cope with systems which won't pass pdata
to the driver.

Tested-by: Paul Zimmerman <paulz@synopsys.com>
Reported-by: Paul Zimmerman <paulz@synopsys.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>