hw/char/bcm2835_aux: Fix assert when receive FIFO fills up

When a bare-metal application on the raspi3 board reads the
AUX_MU_STAT_REG MMIO register while the device's buffer is
at full receive FIFO capacity
(i.e. `s->read_count == BCM2835_AUX_RX_FIFO_LEN`) the
assertion `assert(s->read_count < BCM2835_AUX_RX_FIFO_LEN)`
fails.

Reported-by: Cryptjar <cryptjar@junk.studio>
Suggested-by: Cryptjar <cryptjar@junk.studio>
Resolves: qemu-project/qemu#459


Signed-off-by: Frederik van Hövell <frederik@fvhovell.nl>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
[PMM: commit message tweaks]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>