usb: dwc2: remove early return on clock query

Since we have assigned clk=NULL, which is a valid clk, we should not
be returning when a clock node is not provide. Instead, we should return
only when we cannot enable the clock.

Signed-off-by: Dinh Nguyen <dinguyen@opensource.altera.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>