fix clang -Wfloat-conversion

fix clang -Wfloat-conversion

src/ifinfo.c:358:18: error: implicit conversion turns floating-point number into integer: 'float' to 'uint64_t' (aka 'unsigned long') [-Werror,-Wfloat-conversion]
                        maxtransfer = ceilf((maxbw/8.0f)*interval*1.1f);