input/raw_analog: Fix gcc 7 compiler warnings.

input/raw_analog: Fix gcc 7 compiler warnings.

  ../src/input/raw_analog.c: In function ‘init’:
  ../src/input/raw_analog.c:133:31: warning: ‘%d’ directive output may be truncated writing between 1 and 10 bytes into a region of size 6 [-Wformat-truncation=]
     snprintf(channelname, 8, "CH%d", i + 1);
                                 ^~
  ../src/input/raw_analog.c:133:28: note: directive argument in the range [1, 2147483647]
     snprintf(channelname, 8, "CH%d", i + 1);
                              ^~~~~~
  ../src/input/raw_analog.c:133:3: note: ‘snprintf’ output between 4 and 13 bytes into a destination of size 8
     snprintf(channelname, 8, "CH%d", i + 1);
     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~