[ALSA] usb-audio: add error message about missing split iso support

Modules: USB generic driver

Add an error message for -ENOSYS for situations when split iso support
is needed but not enabled.

Signed-off-by: Clemens Ladisch <clemens@ladisch.de>

sound/usb/usbaudio.c

@@ -788,6 +788,10 @@ static const char *usb_error_string(int err)
 		return "device disabled";
 	case -EHOSTUNREACH:
 		return "device suspended";
+#ifndef CONFIG_USB_EHCI_SPLIT_ISO
+	case -ENOSYS:
+		return "enable CONFIG_USB_EHCI_SPLIT_ISO to play through a hub";
+#endif
 	case -EINVAL:
 	case -EAGAIN:
 	case -EFBIG: