netdev: DM9000: remove BLACKFIN hacking in DM9000 netdev driver

remove integer casting in the read/write IO accessors,
because Blackfin now provides those functions

Tested-by: Javier Herrero <jherrero@hvsistemas.es>
Signed-off-by: Bryan Wu <cooloney@kernel.org>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>