[minor] some comparison warnings between signed and unsigned integer

[hpsaturn]

We have some warnings on:

src/MultiFuncShield.cpp: In member function 'void MultiFuncShield::queueButton(byte)':
src/MultiFuncShield.cpp:308:25: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   if (buttonBufferCount <= sizeof (buttonBuffer))
       ~~~~~~~~~~~~~~~~~~^~~~~~~~~~~
src/MultiFuncShield.cpp: In member function 'void MultiFuncShield::write(const char*, byte)':
src/MultiFuncShield.cpp:496:10: warning: unused variable 'offset' [-Wunused-variable]
     byte offset = 0;
          ^~~~~~
src/MultiFuncShield.cpp:545:22: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     for (int i =0; i < sizeof(displayBuf); i++)
                    ~~^~~~~~~~~
src/MultiFuncShield.cpp:490:35: warning: unused variable 'pBuf' [-Wunused-variable]
   byte displayBuf[] = {0,0,0,0}, *pBuf = displayBuf;
                                   ^~~~
Archiving .pio/build/uno/libac0/libWire.a
src/MultiFuncShield.cpp: In member function 'void MultiFuncShield::isrCallBack()':
src/MultiFuncShield.cpp:777:21: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     for (int i=0; i < sizeof(buttonPins); i++)
                   ~~^~~~~~~~~
src/MultiFuncShield.cpp: In member function 'void MultiFuncShield::manualButtonHandler()':
src/MultiFuncShield.cpp:897:19: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   for (int i=0; i < sizeof(buttonPins); i++)
                 ~~^~~~~~~~~
Indexing .pio/build/uno/lib0a9/libEEPROMEx.a
Indexing .pio/build/uno/libac0/libWire.a
src/MultiFuncShield.cpp: In function 'byte readButton(byte)':
src/MultiFuncShield.cpp:1205:3: warning: control reaches end of non-void function [-Wreturn-type]
   }
   ^