usb: dwc3: of-simple: Fix warning during unbind

In dwc3_of_simple_remove() we are using clk_unprepare() before doing
any clk_disable(). If we enable Common CLK framework (CCF) and try to
unbind dwc3-of-simple driver, we see kernel WARN messages.

This patch fixes this kernel warning by using clk_disable_unprepare()
instead of clk_unprepare().

Signed-off-by: Anurag Kumar Vulisha <anuragku@xilinx.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>