mtd: map: BUG() in non handled cases

Several map-related functions look like a serie of ifs, checking
widths of map. Those functions do not have any handling for default
case. Instead of fiddling with uninitialized_var in those functions,
let's just add a (correct) BUG() to the default case on those maps. This
will also allow us to catch potential errors in maps setup in future.

Signed-off-by: Dmitry Eremin-Solenikov <dmitry_eremin@mentor.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>