serial: amba-pl011: factor out code for writing LCR_H register

The code to cope with a split tx/rx LCR_H register is non-trivial
so put it into it's own function to avoid duplication.

Signed-off-by: Jon Medhurst <tixy@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>