dm raid: remove bogus const from decipher_sync_action() return type

With gcc-4.1.2:

    drivers/md/dm-raid.c:3357: warning: type qualifiers ignored on function return type

Remove the "const" keyword to fix this.

Fixes: 36a240a7 ("dm raid: fix RAID leg rebuild errors")
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Heinz Mauelshagen <heinzm@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>