ixgbe: fix crash with empty VF macvlan list

The adapter->vf_mvs.l list needs to be initialized even if the list is
empty.  Otherwise it will lead to crashes.

Fixes: a1cbb15c

 ("ixgbe: Add macvlan support for VF")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Simon Horman <horms@kernel.org>
Reviewed-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Link: https://lore.kernel.org/r/ZSADNdIw8zFx1xw2@kadam
Signed-off-by: Paolo Abeni <pabeni@redhat.com>