IAR doesn't support __attribute__((fallthrough))

IAR generates warning Pa167 'the "fallthrough" attribute is not supported'.
It doesn't generate warnings when one switch case falls through to another,
so simply make TU_ATTR_FALLTHROUGH expand to an empty string.

Also replace one instance of __attribute__ with the macro.

src/host/usbh.c

@@ -1273,7 +1273,7 @@ static void process_enumeration(tuh_xfer_t* xfer)
         break;
       }
       #endif
-      __attribute__((fallthrough));
+      TU_ATTR_FALLTHROUGH;
 #endif
 
     case ENUM_SET_ADDR: