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.

Author: Ben Avison

src/common/tusb_compiler.h

@@ -189,7 +189,7 @@
   #define TU_ATTR_DEPRECATED(mess)      __attribute__ ((deprecated(mess))) // warn if function with this attribute is used
   #define TU_ATTR_UNUSED                __attribute__ ((unused))           // Function/Variable is meant to be possibly unused
   #define TU_ATTR_USED                  __attribute__ ((used))             // Function/Variable is meant to be used
-  #define TU_ATTR_FALLTHROUGH           __attribute__((fallthrough))
+  #define TU_ATTR_FALLTHROUGH
 
   #define TU_ATTR_PACKED_BEGIN
   #define TU_ATTR_PACKED_END

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: