hw/char/bcm2835_aux: Really use RX FIFO depth

hw/char/bcm2835_aux: Really use RX FIFO depth

While we model a 8-elements RX FIFO since the BCM2835 AUX model
was introduced in commit 97398d900ca ("bcm2835_aux: add emulation
of BCM2835 AUX block") 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>
Message-Id: <20250220092903.3726-6-philmd@linaro.org>