Fix wrong format string type in send_sptps_tcppacket().

This issue was found through a clang-3.7 warning:

protocol_misc.c:167:46: error: format specifies type 'short' but the argument has type 'int'
      [-Werror,-Wformat]
        if(!send_request(c, "%d %hd", SPTPS_PACKET, len))
                                ~~~                 ^~~
                                %d