Ml2: Set correct nullable for admin_state_up

In migrations 492a106273f8_brocade_ml2_mech_dri was set incorrect
nullable parameter:

Set admin_state_up NOT NULL in ml2_brocadeports.

Partial-Bug: #1296282

Change-Id: I051e37990ae3371978eeb1ff71e70ae2a44097bc
Merge "Ml2: Set correct nullable for admin_state_up"