gtphub: fix compilation with gcc 10.2.0

gtphub.c:2915:2: error: ‘snprintf’ argument 4 may overlap destination object ‘buf’ [-Werror=restrict]
 2915 |  snprintf(pos, len, " port %s", portbuf);
      |  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Be better safe and use the stack instead of byte counting in the buffer.

Change-Id: Ied9665ce6bd2633797bbc3a2171e911ada357a22