macloader: Rework mac address half classification

 * Don't return in the middle of a function, set the
   return code and jump to the exit point instead.
 * Close file pointers properly.
 * Unify error code returns (macloader will always exit
   with -1 on error now).

Change-Id: I32ac8ea85fcbabed45550a4d176ac999eff9f9e9