hw/char/imx_serial: Really use RX FIFO depth

hw/char/imx_serial: Really use RX FIFO depth

While we model a 32-elements RX FIFO since the IMX serial
model was introduced in commit 988f2442971 ("hw/char/imx_serial:
Implement receive FIFO and ageing timer") 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>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Tested-by: Bernhard Beschow <shentey@gmail.com>
Message-Id: <20250220092903.3726-7-philmd@linaro.org>