serial: 8250_dw: Disable clock on error

If there is no clock rate for uartclk defined, disable the previously
enabled clock again.

Found by Linux Driver Verification project (linuxtesting.org).

Fixes: 23f5b3fd serial: 8250_dw: only get the clock rate in one place
Signed-off-by: Stefan Potyra <Stefan.Potyra@elektrobit.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>