meta-flow: Simplify mf_from_ofp_port_string()

ofputil_port_from_string() does all the work already.

Signed-off-by: Jarno Rajahalme <jarno.rajahalme@nsn.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>
Merge commit '33ab38d9b861ab07fc4a291b77ce426bb3a1609e' into bsd-port-threaded