iavf: Fix displaying queue statistics shown by ethtool

Driver provided too many lines as an output to ethtool -S command.
Return actual length of string set of ethtool stats. Instead of predefined
maximal value use the actual value on netdev, iterate over active queues.
Without this patch, ethtool -S report would produce additional
erroneous lines of queues that are not configured.

Signed-off-by: Witold Fijalkowski <witoldx.fijalkowski@intel.com>
Signed-off-by: Przemyslaw Patynowski <przemyslawx.patynowski@intel.com>
Signed-off-by: Mateusz Palczewski <mateusz.palczewski@intel.com>
Signed-off-by: Jedrzej Jagielski <jedrzej.jagielski@intel.com>
Tested-by: Konrad Jankowski <konrad0.jankowski@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>