i40e: clear mac filter count on reset

When a VF is reset, it gets a new VSI, so all of its MAC filters go
away. Correctly set the number of filters to 0 when freeing VF
resources. This corrects a problem with failure to add filters when the
VF driver is reloaded.

Change-ID: I2acbecf734287b67473bb225293e14b5096acbef
Signed-off-by: Mitch Williams <mitch.a.williams@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>