netdev: Allow netdev_change_seq_changed() to accept const pointers.

This fixes the following warning in the DPDK code.

../lib/netdev-dpdk.c:790:31: error: passing 'const struct netdev *' to
parameter of type 'struct netdev *' discards qualifiers

Signed-off-by: Ethan Jackson <ethan@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>