mtd: cleanup last uses of MTD_DEBUG config macros

Some messages that were tied to CONFIG_MTD_DEBUG_VERBOSE can now simply
be enabled using dynamic debugging features, if necessary. There's no
need for special debugging functions here.

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