n64: store dev instance into disk private data

The device instance is declared globally. Remove global variable & use
the disk->private_data to store the device instance in the
n64cart_probe() and get the same instance from bio->bi_disk->private
data in n64cart_submit_bio.

Signed-off-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Reviewed-by: Lauri Kasanen <cand@gmx.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>