i40e: add a helper function to validate a VF based on the vf id

When we are trying to change VF settings, it is possible for 2 race
conditions to happen. One, when the VF is created but not yet enabled.
Second, the VF is enabled but the VSI is still not created or not yet
re-created in the VF reset flow.

This patch introduces a helper function to validate that the VF is
enabled and that the VSI is set up. This patch also calls this
function from other functions which could get into these race conditions.
While we are poking around here, remove unnecessary parenthesis that
checkpatch was complaining about.

Signed-off-by: Harshitha Ramamurthy <harshitha.ramamurthy@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>