8250: use callbacks to access UART_DLL/UART_DLM

8250: use callbacks to access UART_DLL/UART_DLM

Some UART HW has a single register combining UART_DLL/UART_DLM
(this was probably forgotten in the change that introduced the
callbacks, commit b83f9003042a73154b800e8fae8a035b9644faa8)

Fixes: b83f9003042a ("[SERIAL] 8250: set divisor register correctly ...")
Signed-off-by: Sebastian Frias <sf84@laposte.net>
Reviewed-by: Peter Hurley <peter@hurleysoftware.com>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>