auxdisplay: ht16k33: Add default to switch-cases

Currently the compiler (GCC) is able to figure out that there is no
other choices possible than those that are already listed in the
switch-cases. However, if we want to move some code to the callback,
compiler will start complaining that no default is defined. Make
sure we have all switch-cases equiped with default.

Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
Reviewed-by: Geert Uytterhoeven <geert@linux-m68k.org>
Tested-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>