clk: mediatek: clk-apmixed: Add helper function to unregister ref2usb_tx

The ref2usb_tx clock was introduced a long time ago and, at that time,
the MediaTek clock drivers were using CLK_OF_DECLARE, so they would
never unregister.

Nowadays, unregistering clock drivers is a thing, as we're registering
them as platform_driver and allowing them to be kernel modules: add a
helper function to cleanup the ref2usb_tx clock during error handling
and upon module removal.

Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Reviewed-by: Chen-Yu Tsai <wenst@chromium.org>
Link: https://lore.kernel.org/r/20220921091455.41327-8-angelogioacchino.delregno@collabora.com


Signed-off-by: Chen-Yu Tsai <wenst@chromium.org>