usb: musb: only set test mode once

The MUSB test mode register can only be set once, otherwise the result
is undefined.

This prevents the debugfs testmode entry to set the register more than
once which causes test failure.
Signed-off-by: Bin Liu <b-liu@ti.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>

drivers/usb/musb/musb_debugfs.c

@@ -191,9 +191,16 @@ static ssize_t musb_test_mode_write(struct file *file,
 {
 	struct seq_file		*s = file->private_data;
 	struct musb		*musb = s->private;
-	u8			test = 0;
+	u8			test;
 	char			buf[18];
 
+	test = musb_readb(musb->mregs, MUSB_TESTMODE);
+	if (test) {
+		dev_err(musb->controller, "Error: test mode is already set. "
+			"Please do USB Bus Reset to start a new test.\n");
+		return count;
+	}
+
 	memset(buf, 0x00, sizeof(buf));
 
 	if (copy_from_user(buf, ubuf, min_t(size_t, sizeof(buf) - 1, count)))