From: Francois Romieu <romieu@fr.zoreil.com>

- more #define for specific bits;
- more scc_patchl use;
- just say no to bozo programming:
  + SCC core _really_ disabled at startup;
  + Interrupts Mask Register setup and SCC core activation are
    done as late as possible (i.e. in dscc4_open());
  + they are reverted if dscc4_open() fails;
  + as well as unconditionnaly in dscc4_close();
- more or less paranoid quirk in Xpr handler.