also handle ZLP in USB_Recv

The fix in arduino/Arduino#6886 is incomplete.
`USB_Recv` also needs to handle ZLPs.

Signed-off-by: Taylor Yu <code@argon.blue>

Author: tlyu

avr/cores/keyboardio/USBCore.cpp

@@ -245,6 +245,10 @@ int USB_Recv(u8 ep, void* d, int len)
 
 	LockEP lock(ep);
 	u8 n = FifoByteCount();
+	if (!n && HasOUT()) {
+		ReleaseRX(); // handle ZLP
+		n = FifoByteCount();
+	}
 	len = min(n,len);
 	n = len;
 	u8* dst = (u8*)d;