i2c: mediatek: Fix an error handling path in mtk_i2c_probe()

The clsk are prepared, enabled, then disabled. So if an error occurs after
the disable step, they are still prepared.

Add an error handling path to unprepare the clks in such a case, as already
done in the .remove function.

Fixes: 8b4fc246

 ("i2c: mediatek: Optimize master_xfer() and avoid circular locking")
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Reviewed-by: Qii Wang <qii.wang@mediatek.com>
Signed-off-by: Wolfram Sang <wsa@kernel.org>