net: zperf: Fix wrong throughput in long-duration traffic test

The zperf received or sent bytes length and duration are in 32bits,
if running long-duration zperf test more than 20min, the value will
overflow, and the test result is wrong. Change it to 64bits can fix
this issue.

Signed-off-by: Maochen Wang <maochen.wang@nxp.com>