[flow][statistics] fixup Port Number
[flow][statistics] fixup Port Number

Port should be a string represented by digit only.
Gopacket by default (via String()) return the service name as well like :
"80(http)"

skydive-project/skydive,lebauce/skydive,fdebonneval/skydive,redhat-cip/skydive,lebauce/skydive,safchain/skydive,safchain/skydive,skydive-project/skydive,safchain/skydive,skydive-project/skydive,safchain/skydive,skydive-project/skydive,skydive-project/skydive,lebauce/skydive,lebauce/skydive,skydive-project/skydive,fdebonneval/skydive,redhat-cip/skydive,lebauce/skydive,lebauce/skydive,fdebonneval/skydive,skydive-project/skydive,redhat-cip/skydive,safchain/skydive,safchain/skydive,safchain/skydive,lebauce/skydive,lebauce/skydive,safchain/skydive