cleanup: Remove return value from MMU2Serial::write()

Removes a missing return warning.

Looks like we don't handle overflow at the moment and in all surrounding
code, so let's remove the return value for now.

Firmware/mmu2_serial.cpp

@@ -19,7 +19,7 @@ void MMU2Serial::flush() {
     // @@TODO - clear the output buffer
 }
 
-size_t MMU2Serial::write(const uint8_t *buffer, size_t size) {
+void MMU2Serial::write(const uint8_t *buffer, size_t size) {
     while(size--){
         fputc(*buffer, uart2io);
         ++buffer;

Firmware/mmu2_serial.h

@@ -12,7 +12,7 @@ public:
     void close();
     int read();
     void flush();
-    size_t write(const uint8_t *buffer, size_t size);
+    void write(const uint8_t *buffer, size_t size);
 };
 
 extern MMU2Serial mmu2Serial;