net/fm10k/base: use different name for override bit

net/fm10k/base: use different name 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: Qi Zhang <qi.z.zhang@intel.com>