[MTD] Use list_for_each_entry[_safe] where appropriate.

Janitorial work to remove temporary pointers and make some functions a bit
more readable.

Signed-off-by: Chris Malley <mail@chrismalley.co.uk>
Reviewed-By: Jörn Engel <joern@logfs.org>
Signed-off-by: David Woodhouse <dwmw2@infradead.org>