Fix minimum latency

Fix minimum latency

Revert back part of commit 9fa316261d7d9
The function abs_diff was erroneously changed to diff_or_zero.
This function was supposed to measure the difference between rx and
tx time; when rx time overflowed and tx time not yet (i.e.
rx time ~= 0 and tx_time ~=UINT32_MAX, this function added UINT32_MAX
to rx_time. The name of the function was confusing and caused the
previous commit. Net effect of previous commit was that every four seconds
the minimum latency was 0
This commit reverse back to the original behavior, with a function name
diff_time.

Change-Id: Ia1b80e48a756cf5df411dcf58ca1cbc835214d13
Signed-off-by: Xavier Simonart <xavier.simonart@intel.com>