ovs-router: Fix sparse warning

Fixes following warning:
../lib/ovs-router.c:162:11: warning: incorrect type in assignment
(different base types)
../lib/ovs-router.c:162:11:    expected restricted ovs_be32
[usertype] <noident>
../lib/ovs-router.c:162:11:    got restricted ovs_be16

Reported-by: Ben Pfaff <blp@nicira.com>
Signed-off-by: Pravin B Shelar <pshelar@nicira.com>
Acked-by: Jarno Rajahalme <jrajahalme@nicira.com>