[ot] hw/opentitan: ot_spi_host: fix STATUS.active bit behavior

Documentation was misleading: "the command is complete when STATUS.ACTIVE goes low."
It turns out STATUS.active goes low when the last pushed command is completed,
in other words, STATUS.active is not released as soon as a command is complete.

Signed-off-by: Emmanuel Blot <eblot@rivosinc.com>

hw/opentitan/ot_spi_host.c

@@ -884,8 +884,10 @@ static void ot_spi_host_post_fsm(void *opaque)
         /* retire command */
         cmdfifo_pop(s->cmd_fifo, NULL);
 
-        /* "the command is complete when STATUS.ACTIVE goes low." */
-        s->fsm.active = false;
+        /* release active only if the last pushed command has been retired */
+        if (cmdfifo_is_empty(s->cmd_fifo)) {
+            s->fsm.active = false;
+        }
     }
 
     ot_spi_host_update_regs(s);