mtd: ixp4xx: fix reading from half-word boundary

Fix handling of reads that don't start on a half-word boundary.

Signed-off-by: Jon Ringle <jon@ringle.org>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>