samples: net: zperf: Convert to use usec directly

Currently the code uses hw cycles and tries to convert them
to usec. I noticed some failures with this, for example the test
duration was sometimes missed meaning that instead of testing 5
seconds, we bailed out after 2 sec etc. After the kernel k_timeout_t
changes, which added APIs to support usec accuracy, we can use usec
and ticks here. This simplifies the code a bit.

Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>