drivers/net/dnet.c: Fix compile warning

Fix this:
dnet.c: In function 'dnet_eth_initialize':
dnet.c:380:15: warning: assignment from incompatible pointer type

Signed-off-by: Joe Hershberger <joe.hershberger@ni.com>
Cc: Joe Hershberger <joe.hershberger@gmail.com>