MTD: fix all kernel-doc warnings

Fix all kernel-doc warnings in MTD headers and source files:
- add some missing struct fields;
- correct some function parameter names;
- use kernel-doc format for function doc. headers;
- nand_ecc.c contains only exported interfaces, no internal ones;

Signed-off-by: Randy Dunlap <rdunlap@xenotime.net>
Signed-off-by: David Woodhouse <dwmw2@infradead.org>