[MTD] replace MTD_ROM with MTD_GENERIC_TYPE

No mtd user should ever check for the device type.  Instead, device features
should be checked by the flags - if at all.
As a first step towards type removal, change MTD_ROM into MTD_GENERIC_TYPE.

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