test: dm: usb: use the real device name

"keyb" is not the real device name, "keyb@3" is.
Signed-off-by: NPeng Fan <peng.fan@nxp.com>
Reviewed-by: NBin Meng <bmeng.cn@gmail.com>

test/dm/usb.c

@@ -125,7 +125,7 @@ static int dm_test_usb_keyb(struct unit_test_state *uts)
 	/* Initially there should be no characters */
 	ut_asserteq(0, tstc());
 
-	ut_assertok(uclass_get_device_by_name(UCLASS_USB_EMUL, "keyb",
+	ut_assertok(uclass_get_device_by_name(UCLASS_USB_EMUL, "keyb@3",
 					      &dev));
 
 	/*