hw/char/mcf_uart: Really use RX FIFO depth

hw/char/mcf_uart: Really use RX FIFO depth

While we model a 4-elements RX FIFO since the MCF UART model
was introduced in commit 20dcee94833 ("MCF5208 emulation"),
we only read 1 char at a time!

Have the IOCanReadHandler handler return how many elements are
available, and use that in the IOReadHandler handler.

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Luc Michel <luc.michel@amd.com>
Tested-by: Thomas Huth <huth@tuxfamily.org>
Reviewed-by: Thomas Huth <huth@tuxfamily.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20250220092903.3726-9-philmd@linaro.org>