m68k: mac: Don't send IOP message until channel is idle

In the following sequence of calls, iop_do_send() gets called when the
"send" channel is not in the IOP_MSG_IDLE state:

	iop_ism_irq()
		iop_handle_send()
			(msg->handler)()
				iop_send_message()
			iop_do_send()

Avoid this by testing the channel state before calling iop_do_send().

When sending, and iop_send_queue is empty, call iop_do_send() because
the channel is idle. If iop_send_queue is not empty, iop_do_send() will
get called later by iop_handle_send().

Fixes: 1da177e4 ("Linux-2.6.12-rc2")
Signed-off-by: Finn Thain <fthain@telegraphics.com.au>
Tested-by: Stan Johnson <userm57@yahoo.com>
Cc: Joshua Thompson <funaho@jurai.org>
Link: https://lore.kernel.org/r/6d667c39e53865661fa5a48f16829d18ed8abe54.1590880333.git.fthain@telegraphics.com.auSigned-off-by: Geert Uytterhoeven <geert@linux-m68k.org>

arch/m68k/mac/iop.c

@@ -415,7 +415,8 @@ static void iop_handle_send(uint iop_num, uint chan)
 	msg->status = IOP_MSGSTATUS_UNUSED;
 	msg = msg->next;
 	iop_send_queue[iop_num][chan] = msg;
-	if (msg) iop_do_send(msg);
+	if (msg && iop_readb(iop, IOP_ADDR_SEND_STATE + chan) == IOP_MSG_IDLE)
+		iop_do_send(msg);
 }
 
 /*
@@ -489,16 +490,12 @@ int iop_send_message(uint iop_num, uint chan, void *privdata,
 
 	if (!(q = iop_send_queue[iop_num][chan])) {
 		iop_send_queue[iop_num][chan] = msg;
+		iop_do_send(msg);
 	} else {
 		while (q->next) q = q->next;
 		q->next = msg;
 	}
 
-	if (iop_readb(iop_base[iop_num],
-	    IOP_ADDR_SEND_STATE + chan) == IOP_MSG_IDLE) {
-		iop_do_send(msg);
-	}
-
 	return 0;
 }