wifi: mt76: mt7915: clear wcid mask under mutex after RCU pointer clear

In the Linux kernel, the following vulnerability has been resolved: wifi: mt76: mt7915: clear wcid mask under mutex after RCU pointer clear mt7915_remove_interface() cleared the wcid mask bit with no lock held and before clearing the RCU wcid pointer. The mask is a non-atomic RMW shared with the allocators, which all run under dev->mt76.mutex; on DBDC the two wiphys share one mt76_dev, so this raced add_interface/sta_add on the other band and could leak or double-hand-out a wcid. Clearing the bit before the RCU pointer also let a concurrent allocation reuse the index and publish its wcid, which the subsequent NULL assignment then wiped. Move the clear into the existing mutex section, after the RCU pointer is cleared.

Where it's fixed