svcauth_gss: replace a trivial 'switch' with an 'if'

Code like:

  switch(xxx) {
  case -error1:
  case -error2:
     ..
     return;
  case 0:
     stuff;
  }

  can more naturally be written:

  if (xxx < 0)
      return;

  stuff;

Signed-off-by: NeilBrown <neilb@suse.de>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>