fm10k: use different name than FM10K_VLAN_CLEAR for override bit

Use a new #define FM10K_VLAN_OVERRIDE even though we're using the exact
same bit. The reason for this is clarity in the code, otherwise you can
read FM10K_VLAN_CLEAR and think it should be removed. Also add a comment
explaining why the FM10K_VLAN_OVERRIDE bit is set.

Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Tested-by: Krishneil Singh <Krishneil.k.singh@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>