i40evf: Fixed guest OS panic when removing vf driver

Removing VF driver during device still in reset caused guest OS panic.

in the i40evf_remove(), we're trying to clean mac_filter_list which has
not been initialized since the device is still stuck at the reset.
The change is to initialize the filter_list before setting any task.

Change-ID: I8b59df7384416c7e6f2d264b598f447e1c2c92b0
Signed-off-by: Serey Kong <serey.kong@intel.com>
Tested-by: Sibai Li <sibai.li@intel.com>
Signed-off-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>