usb: dwc2: get optional clock by devm_clk_get_optional()

When the driver tries to get optional clock, it ignores all errors,
but if only ignores -ENOENT, it will cover some real errors, such as
-EPROBE_DEFER, so use devm_clk_get_optional() to get optional clock.

Cc: Minas Harutyunyan <hminas@synopsys.com>
Signed-off-by: Chunfeng Yun <chunfeng.yun@mediatek.com>
Acked-by: Minas Harutyunyan <hminas@synopsys.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>