i40e: Fix crash caused by stress setting of VF MAC addresses

Add update to the VSI pointer passed to the i40e_set_vf_mac function.
If VF is in reset state the driver waits in i40e_set_vf_mac function
for the reset to be complete, yet after reset the vsi pointer
that was passed into this function is no longer valid.

The patch updates local VSI pointer directly from pf->vsi array,
by using the id stored in VF pointer (lan_vsi_idx).

Without this commit the driver might occasionally invoke general
protection fault in kernel and disable the OS entirely.

Signed-off-by: Slawomir Laba <slawomirx.laba@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>