net: shell: Avoid gcc warning print with string catenation

gcc prints this warning message

'strncat' specified bound 1 equals source length [-Wstringop-overflow=]
   58 |                 strncat(fd, "C", 1);

There was no error in the code but avoid the warning by not using
strncat().

Signed-off-by: Jukka Rissanen <jukka.rissanen@nordicsemi.no>