vrrp: Let only base interface change the VMAC interface status flags

The interface status flags for a VMAC interface shall only be changed by the
base interface, never by reading the actual VMAC interface flags.

Signed-off-by: Jonas Johansson <jonasj76@gmail.com>
Merge pull request #85 from jonasj76/vmac-startup

vrrp: Let only base interface change the VMAC interface status flags