iavf: Fix refreshing iavf adapter stats on ethtool request

Currently iavf adapter statistics are refreshed only in a
watchdog task, triggered approximately every two seconds,
which causes some ethtool requests to return outdated values.

Add explicit statistics refresh when requested by ethtool -S.

Fixes: b476b003

 ("iavf: Move commands processing to the separate function")
Signed-off-by: Jan Sokolowski <jan.sokolowski@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>