fix samd21 race condition with setup packet

reproduced with Adafruit_TinyUSB_ArduinoCore port commit 11d669b4d2a40eb2fc5e51b2a9707a6de9d42363 and SAMD BSP 1.6.1

src/portable/microchip/samd/dcd_samd.c

@@ -43,6 +43,8 @@ static inline void prepare_setup(void)
 {
   // Only make sure the EP0 OUT buffer is ready
   sram_registers[0][0].ADDR.reg = (uint32_t) _setup_packet;
+  sram_registers[0][0].PCKSIZE.bit.MULTI_PACKET_SIZE = sizeof(_setup_packet);
+  sram_registers[0][0].PCKSIZE.bit.BYTE_COUNT = 0;
 }
 
 // Setup the control endpoint 0.