serial: 8250-mtk: Fix reference leak in mtk8250_probe

The pm_runtime_enable will increase power disable depth.
Thus a pairing decrement is needed on the error handling
path to keep it balanced according to context.

Fixes: e32a83c7 ("serial: 8250-mtk: modify mtk uart power and clock management")
Signed-off-by: Zhang Qilong <zhangqilong3@huawei.com>
Link: https://lore.kernel.org/r/20201119141126.168850-1-zhangqilong3@huawei.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>