usb: xhci-mtk: get optional clock by devm_clk_get_optional()

Use devm_clk_get_optional() to get optional clock instead of
optional_clk_get() which uses devm_clk_get() to get clock and
checks for -EPROBE_DEFER but not -ENOENT as devm_clk_get_optional()
does, in fact, only ignoring -ENOENT will cover more errors, so the
replacement doesn't change original purpose.

Signed-off-by: Chunfeng Yun <chunfeng.yun@mediatek.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>