mtd: doc: do not initialize mtd_info fields to 0 or NULL

The 'struct mtd_info' object is allocated with 'kzalloc()', so it
contains only zeroes - no need to initialize various fields to 0 or
NULL.

Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>