[MTD] Disconnect struct mtd_info from ABI

mtdchar.c direcly copied part of struct mtd_info to userspace, thereby
implicitly making it part of the ABI.  With this patch, struct
mtd_info is independent of the ABI and can have its fields removed,
reordered, etc.

Signed-off-by: Joern Engel <joern@wh.fh-wedel.de>