Add a function to test if USB interrupt send would block

usb.c

@@ -604,6 +604,11 @@ ISR(USB_COM_vect)
 #endif
 }
 
+char usb_interruptReady(void)
+{
+	return interrupt_data_len == -1;
+}
+
 void usb_interruptSend(void *data, int len)
 {
 	uint8_t sreg = SREG;

usb.h

@@ -198,6 +198,7 @@ struct usb_parameters {
 	struct usb_hid_parameters hid_params[MAX_HID_INTERFACES];
 };
 
+char usb_interruptReady(void);
 void usb_interruptSend(void *data, int len); // EP1
 void usb_init(const struct usb_parameters *params);
 void usb_doTasks(void);