[Hell] Fixed issues found by cppcheck

src/protocols/Hellschreiber/Hellschreiber.cpp

@@ -30,7 +30,7 @@ int16_t HellClient::begin(float base, float rate) {
   return(phyLayer->startDirect());
 }
 
-size_t HellClient::printGlyph(uint8_t* buff) {
+size_t HellClient::printGlyph(const uint8_t* buff) {
   // print the character
   Module* mod = phyLayer->getMod();
   bool transmitting = false;

src/protocols/Hellschreiber/Hellschreiber.h

@@ -117,7 +117,7 @@ class HellClient: public RadioLibPrint {
       \param buff Buffer of pixels to send, in a 7x7 pixel array.
       \returns Always returns the number of printed glyphs (1).
     */
-    size_t printGlyph(uint8_t* buff);
+    size_t printGlyph(const uint8_t* buff);
 
     /*!
       \brief Invert text color.