MN10300: Optimise do_csum()

Optimise do_csum() to gang up the loads so they're less likely to get
interruptions between.

Signed-off-by: Akira Takeuchi <takeuchi.akr@jp.panasonic.com>
Signed-off-by: Kiyoshi Owada <owada.kiyoshi@jp.panasonic.com>
Signed-off-by: David Howells <dhowells@redhat.com>