utils_netperf: Fix self.netperf_path typo issue

When compiling netperf from source, the variable to be set in the Client
class should be netperf_path and not netserver_path. This fix was done
before pushing the util_netperf feature, but somehow lost.

Signed-off-by: Jonas Eriksson <jonas.eriksson@enea.com>
Merge pull request #1586 from zqad/netperf_fix

utils_netperf: Fix self.netperf_path typo issue