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: https://gitlab.com/qemu-project/qemu/-/issues/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>

hw/char/bcm2835_aux.c

@@ -138,7 +138,7 @@ static uint64_t bcm2835_aux_read(void *opaque, hwaddr offset, unsigned size)
         res = 0x30e; /* space in the output buffer, empty tx fifo, idle tx/rx */
         if (s->read_count > 0) {
             res |= 0x1; /* data in input buffer */
-            assert(s->read_count < BCM2835_AUX_RX_FIFO_LEN);
+            assert(s->read_count <= BCM2835_AUX_RX_FIFO_LEN);
             res |= ((uint32_t)s->read_count) << 16; /* rx fifo fill level */
         }
         return res;