usb: dwc2: gadget: Do not fail probe if there isn't a clock node

Since the dwc2 hcd driver is currently not looking for a clock node during
init, we should not completely fail if there isn't a clock provided.
By assigning clk = NULL, this allows the driver, when configured for dual-role
mode, to be able to continue loading the host portion of the driver when
a clock node is not specified.

Acked-by: Paul Zimmerman <paulz@synopsys.com>
Signed-off-by: Dinh Nguyen <dinguyen@opensource.altera.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>