dm raid: remove unused d variable

clang with W=1 reports
drivers/md/dm-raid.c:2212:15: error: variable
  'd' set but not used [-Werror,-Wunused-but-set-variable]
        unsigned int d;
                     ^
This variable is not used so remove it.

Signed-off-by: Tom Rix <trix@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@kernel.org>