esp: add trivial implementation of the ESP_RFLAGS register

The bottom 5 bits contain the number of bytes remaining in the FIFO which is
trivial to implement with Fifo8 (the remaining bits are unimplemented and left
as 0 for now).

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Message-Id: <20210304221103.6369-41-mark.cave-ayland@ilande.co.uk>

hw/scsi/esp.c

@@ -818,6 +818,10 @@ uint64_t esp_reg_read(ESPState *s, uint32_t saddr)
             val = s->rregs[saddr];
         }
         break;
+     case ESP_RFLAGS:
+        /* Bottom 5 bits indicate number of bytes in FIFO */
+        val = fifo8_num_used(&s->fifo);
+        break;
     default:
         val = s->rregs[saddr];
         break;