EDAC, sb_edac: Simplify switch statement

clang static analyzer reports this problem

sb_edac.c:959:2: warning: Undefined or garbage value
  returned to caller
        return type;
        ^~~~~~~~~~~

This is a false positive.

However by initializing the type to DEV_UNKNOWN the 3 case can be
removed from the switch, saving a comparison and jump.

Signed-off-by: Tom Rix <trix@redhat.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>
Link: https://lore.kernel.org/r/20200907153225.7294-1-trix@redhat.com