mtd: define `mtd_is_*()' functions

These functions can be used instead of referencing -EUCLEAN and -EBADMSG
all over the place. They should help make code a little bit more
readable.

Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@intel.com>