fixed rtl_tcp cases for examples 2,3,4

[andimik]

Fixes the following error in examples

/home/andreas/apps/dab-cmdline/example-3/main.cpp: In function ‘int main(int, char**)’:
/home/andreas/apps/dab-cmdline/example-3/main.cpp:441:20: error: conversion from ‘const char*’ to ‘int’ in a converted constant expression
  441 |               case "G":
      |                    ^~~
/home/andreas/apps/dab-cmdline/example-3/main.cpp:441:20: error: could not convert ‘"G"’ from ‘const char [2]’ to ‘int’
  441 |               case "G":
      |                    ^~~
      |                    |
      |                    const char [2]
make[2]: *** [CMakeFiles/dab-rtl_tcp-3.dir/build.make:76: CMakeFiles/dab-rtl_tcp-3.dir/main.cpp.o] Fehler 1
make[1]: *** [CMakeFiles/Makefile2:84: CMakeFiles/dab-rtl_tcp-3.dir/all] Fehler 2
make: *** [Makefile:136: all] Fehler 2

[JvanKatwijk]

thanks