[AVR] Send USB ZLP if required

Fixes arduino#5732

Author: facchinm

hardware/arduino/avr/cores/arduino/USBAPI.h

@@ -196,7 +196,7 @@ int USB_RecvControl(void* d, int len);
 int USB_RecvControlLong(void* d, int len);
 
 uint8_t	USB_Available(uint8_t ep);
-uint8_t USB_SendSpace(uint8_t ep);
+int8_t USB_SendSpace(uint8_t ep);
 int USB_Send(uint8_t ep, const void* data, int len);	// blocking
 int USB_Recv(uint8_t ep, void* data, int len);		// non-blocking
 int USB_Recv(uint8_t ep);							// non-blocking

hardware/arduino/avr/cores/arduino/USBCore.cpp

@@ -251,14 +251,12 @@ int USB_Recv(u8 ep)
 }
 
 //	Space in send EP
-u8 USB_SendSpace(u8 ep)
+int8_t USB_SendSpace(u8 ep)
 {
 	LockEP lock(ep);
 	if (!ReadWriteAllowed())
-		return 0;
-	// subtract 1 from the EP size to never send a full packet,
-	// this avoids dealing with ZLP's in USB_Send
-	return USB_EP_SIZE - 1 - FifoByteCount();
+		return -1;
+	return USB_EP_SIZE - FifoByteCount();
 }
 
 //	Blocking Send of data to an endpoint
@@ -273,12 +271,15 @@ int USB_Send(u8 ep, const void* d, int len)
 	}
 
 	int r = len;
+
+	bool sendZlp = (len % USB_EP_SIZE) == 0;
+
 	const u8* data = (const u8*)d;
 	u8 timeout = 250;		// 250ms timeout on send? TODO
-	while (len)
+	while (len || sendZlp)
 	{
-		u8 n = USB_SendSpace(ep);
-		if (n == 0)
+		int8_t n = USB_SendSpace(ep);
+		if (n < 0)
 		{
 			if (!(--timeout))
 				return -1;
@@ -293,6 +294,13 @@ int USB_Send(u8 ep, const void* d, int len)
 			// Frame may have been released by the SOF interrupt handler
 			if (!ReadWriteAllowed())
 				continue;
+
+			if (len == 0 && sendZlp) {
+				// empty transfer sent
+				Send8(0);
+				sendZlp = false;
+			}
+
 			len -= n;
 			if (ep & TRANSFER_ZERO)
 			{
@@ -475,7 +483,7 @@ static
 bool SendConfiguration(int maxlen)
 {
 	//	Count and measure interfaces
-	InitControl(0);	
+	InitControl(0);
 	u8 interfaces = SendInterfaces();
 	ConfigDescriptor config = D_CONFIG(_cmark + sizeof(ConfigDescriptor),interfaces);