fel: Add release USB interface to allow proper operation on Mac OS X.

Closes #24. See https://groups.google.com/forum/#!topic/linux-sunxi/sEJ1eTBoCvw

Tested-by: Bernhard Nortmann <bernhard.nortmann@web.de>
Reviewed-by: Siarhei Siamashka <siarhei.siamashka@gmail.com>

fel.c

@@ -1437,6 +1437,7 @@ int main(int argc, char **argv)
 		aw_fel_execute(handle, uboot_entry);
 	}
 
+	libusb_release_interface(handle, 0);
 #if defined(__linux__)
 	if (iface_detached >= 0)
 		libusb_attach_kernel_driver(handle, iface_detached);