openvswitch: fix a sparse warning

Fix a sparse warning introduced by commit:
f5796684 (openvswitch: Add support for
Geneve tunneling.) caught by kbuild test robot:

reproduce:
  # apt-get install sparse
  #   git checkout f5796684


  #     make ARCH=x86_64 allmodconfig
  #       make C=1 CF=-D__CHECK_ENDIAN__
  #
  #
  #       sparse warnings: (new ones prefixed by >>)
  #
  #       >> net/openvswitch/vport-geneve.c:109:15: sparse: incorrect type in assignment (different base types)
  #          net/openvswitch/vport-geneve.c:109:15:    expected restricted __be16 [usertype] sport
  #             net/openvswitch/vport-geneve.c:109:15:    got int
  #             >> net/openvswitch/vport-geneve.c:110:56: sparse: incorrect type in argument 3 (different base types)
  #                net/openvswitch/vport-geneve.c:110:56:    expected unsigned short [unsigned] [usertype] value
  #                   net/openvswitch/vport-geneve.c:110:56:    got restricted __be16 [usertype] sport

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Andy Zhou <azhou@nicira.com>
Signed-off-by: David S. Miller <davem@davemloft.net>