musb_gadget: Fix for spurious interrupts on endpoint zero.

There is a multi-year old bug in the MUSB hardware which is not documented.
It causes spurious interrupts and have various symptoms, like endless
"SetupEnd came in a wrong ep0stage" messages. The fix is taken from the
FreeBSD's musb driver.

How to reproduce:
For example issue clear-stall on a couple of endpoints very fast,
like one request per 125us. After a while the bug triggers and the
musb-chip becomes unusable until next re-enumeration.
Signed-off-by: Hans Petter Selasky <hps@bitfrost.no>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/usb/musb/musb_gadget_ep0.c

@@ -679,6 +679,14 @@ irqreturn_t musb_g_ep0_irq(struct musb *musb)
 			musb_readb(mbase, MUSB_FADDR),
 			decode_ep0stage(musb->ep0_state));
 
+	if (csr & MUSB_CSR0_P_DATAEND) {
+		/*
+		 * If DATAEND is set we should not call the callback,
+		 * hence the status stage is not complete.
+		 */
+		return IRQ_HANDLED;
+	}
+
 	/* I sent a stall.. need to acknowledge it now.. */
 	if (csr & MUSB_CSR0_P_SENTSTALL) {
 		musb_writew(regs, MUSB_CSR0,