dma: don't wait on unbinds in SH3

This can freeze for reasons I didn't completely root cause. The MWE
leading to this was made by @Calamari, is very dependent on factors I
haven't completely characterized, but includes having the DMA driver,
doing a world switch; and seems to require some timers (via gray engine)
and stack usage, but I'm not clear on the role of that.

src/dma/dma.c

@@ -298,6 +298,8 @@ static void configure(void)
 
 static void funbind(void)
 {
+	if(isSH3()) return;
+
 	/* Wait for all OS transfers to finish before taking over */
 	for(int channel = 0; channel < 6; channel++)
 		dma_channel_wait(channel, true);
@@ -305,6 +307,8 @@ static void funbind(void)
 
 static void unbind(void)
 {
+	if(isSH3()) return;
+
 	/* Make sure all DMA transfers are finished before leaving gint */
 	for(int channel = 0; channel < 6; channel++)
 		dma_channel_wait(channel, false);