Simplify test for RAM devices

mtdblock is the only user of aggregate capabilities in mtd.  This is clearly
bogus and should be changed.  In particular, it tries to determine whether
the device in question is a piece of RAM.  For every single driver that fits
the current criteria, an easier test would be to check for the type being
MTD_RAM.

Signed-off-by: Jörn Engel <joern@wohnheim.fh-wedel.de>
Signed-off-by: David Woodhouse <dwmw2@infradead.org>