mtip32xx: fix minor number

When a device is surprise removed and inserted, it is assigned a new minor
number because driver use multiples of 'instance' number. Modified to use the
multiples of 'index' for minor number.

Signed-off-by: Asai Thambi S P <asamymuthupa@micron.com>
Signed-off-by: Jens Axboe <axboe@fb.com>