mtd: Disable mtdchar mmap on MMU systems

commit f5cf8f07423b2677cebebcebc863af77223a4972 upstream.

This code was broken because it assumed that all MTD devices were map-based.
Disable it for now, until it can be fixed properly for the next merge window.

Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>
[bwh: Backported to 3.2: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>