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.