samples: net: zperf: Initialize address family properly

The IP address family was not set correctly when setting up
the connection. This lead to connectivity error in udp.upload
command.

Change-Id: I598ff2675f97e10e2033763a497f7583c94f3840
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>