iavf: extract iavf_watchdog_step() out of iavf_watchdog_task()

Finish up easy refactor of watchdog_task, total for this + prev two
commits is:
 1 file changed, 47 insertions(+), 82 deletions(-)

Signed-off-by: Przemek Kitszel <przemyslaw.kitszel@intel.com>
Tested-by: Rafal Romanowski <rafal.romanowski@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>