vxlan: Use proper endian type for vni in vxlan[6]_xmit_skb

Silences the following sparse warnings:
drivers/net/vxlan.c:1818:21: warning: incorrect type in assignment (different base types)
drivers/net/vxlan.c:1818:21:    expected restricted __be32 [usertype] vx_vni
drivers/net/vxlan.c:1818:21:    got unsigned int [unsigned] [usertype] vni
drivers/net/vxlan.c:2014:58: warning: incorrect type in argument 11 (different base types)
drivers/net/vxlan.c:2014:58:    expected unsigned int [unsigned] [usertype] vni
drivers/net/vxlan.c:2014:58:    got restricted __be32 [usertype] <noident>

Fixes: 614732ea ("openvswitch: Use regular VXLAN net_device device")
Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Thomas Graf <tgraf@suug.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>