mtip32xx: fix two smatch warnings

Dan reports:

New smatch warnings:
drivers/block/mtip32xx/mtip32xx.c:2728 show_device_status() warn: variable dereferenced before check 'dd' (see line 2727)
drivers/block/mtip32xx/mtip32xx.c:2758 show_device_status() warn: variable dereferenced before check 'dd' (see line 2757)

which are checking if dd == NULL, in a list_for_each_entry() type loop.
Get rid of the check, dd can never be NULL here.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>