ixgbevf: add disable link state

Add possibility to disable link state if it is administratively
disabled in PF.

It is part of the general functionality that allows the PF driver
to control the state of the virtual link VF devices.

Signed-off-by: Slawomir Mrozowicz <slawomirx.mrozowicz@intel.com>
Tested-by: Konrad Jankowski <konrad0.jankowski@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>