Increase MMU2 serial buffer size with USB serial (#15646)

Marlin/src/feature/prusa_MMU2/mmu2.cpp

 volatile bool MMU2::finda_runout_valid;
 int16_t MMU2::version = -1, MMU2::buildnr = -1;
 millis_t MMU2::last_request, MMU2::next_P0_request;
-char MMU2::rx_buffer[16], MMU2::tx_buffer[16];
+char MMU2::rx_buffer[MMU_RX_SIZE], MMU2::tx_buffer[MMU_TX_SIZE];
 
 #if HAS_LCD_MENU && ENABLED(MMU2_MENUS)
 

Marlin/src/feature/prusa_MMU2/mmu2.h

   #include "../runout.h"
 #endif
 
+#if SERIAL_USB
+  #define MMU_RX_SIZE 256
+  #define MMU_TX_SIZE 256
+#else
+  #define MMU_RX_SIZE  16
+  #define MMU_TX_SIZE  16
+#endif
+
 struct E_Step;
 
 class MMU2 {
   static volatile bool finda_runout_valid;
   static int16_t version, buildnr;
   static millis_t last_request, next_P0_request;
-  static char rx_buffer[16], tx_buffer[16];
+  static char rx_buffer[MMU_RX_SIZE], tx_buffer[MMU_TX_SIZE];
 
   static inline void set_runout_valid(const bool valid) {
     finda_runout_valid = valid;