EHCI: Simplify short packet logic to use alternate pointer.

The alternate pointer is the software equivalent of the alternate
physical address in the hardware queue head part. The controller
follows that path, so we can follow the alternate pointer as well.
Simplifies and generalizes the logic.

src/add-ons/kernel/busses/usb/ehci.cpp

@@ -1646,19 +1646,16 @@ EHCI::FinishTransfers()
 				if (((status >> EHCI_QTD_PID_SHIFT) & EHCI_QTD_PID_MASK)
 						== EHCI_QTD_PID_IN
 					&& ((status >> EHCI_QTD_BYTES_SHIFT) & EHCI_QTD_BYTES_MASK)
-						!=0) {
-					// a short packet condition existed on this descriptor
-					if ((transfer->transfer->TransferPipe()->Type()
-						& USB_OBJECT_CONTROL_PIPE) != 0) {
-						// for control pipes, the next descriptor
-						// executed is the Status descriptor
-						while (!(descriptor->next_phy & EHCI_ITEM_TERMINATE)) {
-							descriptor = descriptor->next_log;
-						}
+						!= 0) {
+					// a short packet condition existed on this descriptor,
+					// follow the alternate next pointer if set
+					if (descriptor->alt_next_log != NULL) {
+						descriptor = descriptor->alt_next_log;
 						continue;
 					}
-					// for bulk/interrupt pipes, no other descriptors are
-					// executed
+
+					// no alternate next, transfer is done
+					callbackStatus = B_OK;
 					transferDone = true;
 					break;
 				}

src/add-ons/kernel/busses/usb/ehci_hardware.h

@@ -207,7 +207,7 @@ typedef struct ehci_qtd {
 	// Software Part
 	uint32		this_phy;
 	struct ehci_qtd	*next_log;
-	void		*alt_next_log;
+	struct ehci_qtd	*alt_next_log;
 	size_t		buffer_size;
 	void		*buffer_log;
 } _PACKED ehci_qtd;