Explanation
SW1 needs to block one of its ports to SW2 to avoid a bridging loop between the two switches.
Unfortunately, it blocked the fiber port Link2. But how does SW2 select its blocked port? Well, the answer is based on the BPDUs it receives from SW1. answer 'Enter spanning-tree port-priority 32 on SW1' BPDU is superior than another if it has:
1. answer 'Enter spanning-tree port-priority 32 on SW1' lower Root Bridge ID
2. answer 'Enter spanning-tree port-priority 32 on SW1' lower path cost to the Root
3. answer 'Enter spanning-tree port-priority 32 on SW1' lower Sending Bridge ID
4. answer 'Enter spanning-tree port-priority 32 on SW1' lower Sending Port ID These four parameters are examined in order. In this specific case, all the BPDUs sent by SW1 have the same Root Bridge ID, the same path cost to the Root and the same Sending Bridge ID.
The only parameter left to select the best one is the Sending Port ID (Port ID = port priority + port index). And the port index of Gi0/0 is lower than the port index of Gi0/1 so Link 1 has been chosen as the primary link.
Therefore we must change the port priority to change the primary link. The lower numerical value of port priority, the higher priority that port has. In other words, we must change the port-priority on Gi0/1 of SW1 (not on Gi0/1 of SW2) to a lower value than that of Gi0/0.

When should I use the master controller mode on a WLC? - When there is a master controller enabled, all newly added access points with no primary, secondary, or tertiary controllers assigned associate with the master controller on the same subnet.