usb: dwc3: fix overriding core clock rate to default max

Fix the bug introduced by commit 82d4ec97
("usb: Add support for reset controller framework") which
overrides the core clock rate from device tree.

Change-Id: Ic3ef2229fa8552301e09dfb912e79e044a81324f
Signed-off-by: Vamsi Krishna Samavedam <vskrishn@codeaurora.org>