mtd: ixp4xx map: use resource_size

Use the resource_size inline function instead of manually calculating
the resource size. This reduces the chance of introducing off-by-one
errors.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>