Use linked list for device's drivers, not fixed size array

USBHost.h

 	Pipe_t   *control_pipe;
 	Device_t *next;
 	setup_t  setup;
-	USBHostDriver *driver[6];
+	//USBHostDriver *driver[6];
+	USBHostDriver *drivers;
 	uint8_t  speed; // 0=12, 1=1.5, 2=480 Mbit/sec
 	uint8_t  address;
 	uint8_t  hub_address;

enumeration.cpp

 			return;
 		case 15: // control transfers for other stuff?
 			// TODO: handle other standard control: set/clear feature, etc
-			for (unsigned int i=0; i < 6; i++) {
-				if (dev->driver[i] == NULL) break; // no more drivers
-				if (dev->driver[i]->control_callback(transfer)) {
+			for (USBHostDriver *d = dev->drivers; d != NULL; d = d->next) {
+				if (d->control_callback(transfer)) {
 					// this driver processed the control transfer reply
 					return;
 				}
 				available_drivers = driver->next;
 			}
 			driver->next = NULL;
-			dev->driver[0] = driver;
+			dev->drivers = driver;
 			return;
 		}
 		prev = driver;