instead of converting to int and back

Fixes clang warnings of the form:
HVC.c:190:43: warning: implicit conversion changes signedness: 'int' to
      'unsigned long' [-Wsign-conversion]
          if (strncmp(spec, _XcmsTekHVC_prefix, n) != 0) {
              ~~~~~~~
Signed-off-by: Alan Coopersmith <alan.coopersmith@oracle.com>
Backported-to-NX-by: Ulrich Sibiller <uli42@gmx.de>