hw/char/pl011: Warn when using disabled transmitter

hw/char/pl011: Warn when using disabled transmitter

We shouldn't transmit characters when the full UART or its
transmitter is disabled. However we don't want to break the
possibly incomplete "my first bare metal assembly program"s,
so we choose to simply display a warning when this occurs.

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20240719181041.49545-9-philmd@linaro.org>