128: Hex metadata is being displayed in decimal in flow statistics

Fixed the code to use hex format specifier. 

After fix:
> $ sudo dpctl unix:/tmp/s1 flow-mod cmd=add,table=5 in_port=2,meta=0xb 
> SENDING (xid=0xF0FF00F0):
> flow_mod{table="5", cmd="add", cookie="0x0", mask="0x0", idle="0", hard="0", prio="32768", buf="none", port="any", group="any", flags="0x0", match=oxm{in_port="2", metadata="0xb"}, insts=[]}

> OK.

> $ sudo dpctl unix:/tmp/s1 stats-flow | grep "{table=\"5\""
> stat_repl{type="flow", flags="0x0", stats=[{table="5", match="oxm{in_port="2", metadata="0xb"}", dur_s="6", dur_ns="672000", prio="32768", idle_to="0", hard_to="0", cookie="0x0", pkt_cnt="0", byte_cnt="0", insts=[]}]}