dnet: fix wrong use of platform_set_drvdata()

platform_set_drvdata() was used with argument of incorrect type and
could cause memory corruption. Moreover, because of not setting drvdata
in the correct place not all resources were freed upon module unload.

Signed-off-by: Ilya Yanok <yanok@emcraft.com>
Signed-off-by: David S. Miller <davem@davemloft.net>