usb: dwc3: gadget: Fix dead code writing GCTL.RAMCLKSEL

The register programming code in dwc2_updated_ram_clk_sel() will never
be executed. And in fact the entire function can be removed as there is
no way to override the default value of GCTL.RAMCLKSEL. Remove the
function and add a comment explaining where GCTL.RAMCLKSEL should be
programmed if needed in the future.

This fixes dead code warnings in coverity.

Signed-off-by: John Youn <johnyoun@synopsys.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>