Call mutex_unlock(&drv->reg_lock) before returning the error code.

Link: https://lore.kernel.org/all/1bada9b2-d276-4123-bfdf-03d165569543@moroto.mountain/
Fixes: d2805601 ("PM / devfreq: mediatek: protect oop in critical session")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>