usb: misc: usb3503: get optional clock by devm_clk_get_optional()
When the driver tries to get optional clock, it ignores all errors except
-EPROBE_DEFER, but if only ignores -ENOENT, it will cover some real errors,
such as -ENOMEM, so use devm_clk_get_optional() to get optional clock.
And remove unnecessary stack variable clk.