[MTD] Use single flag to mark writeable devices.

Two flags exist to decide whether a device is writeable or not.  None of
those two flags is checked for independently, so they are clearly redundant,
if not an invitation to bugs.  This patch removed both of them, replacing
them with a single new flag.

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