samples: zperf: cleanup types for constant arrays

There are a few arrays we use with print_number and parse_number.  These
functions expect a const uint32_t so lets have the arrays for TIME_US,
KBPS, and K match.

Change-Id: I6347b06af2374d702144084bb6b538a478fb8fac
Signed-off-by: Kumar Gala <kumar.gala@linaro.org>