wifi: mt76: mt7615: avoid waiting for mac work under the mt76 mutex

In the Linux kernel, the following vulnerability has been resolved: wifi: mt76: mt7615: avoid waiting for mac work under the mt76 mutex mt7615_suspend() acquired the mt76 mutex and then called cancel_delayed_work_sync() on mac_work. mt7615_mac_work() acquires the same mutex via mt7615_mutex_acquire() at the top of the worker, so if mac_work is already running and blocked on the mutex, the suspend path deadlocks waiting for the work it holds the mutex against. Flush scan_work and mac_work before taking the mutex, matching the suspend paths in mt7921 and mt7925. scan_work only takes the mt76 spinlock, but moving it keeps the sequence consistent. This also keeps mac_work from running over an already suspended HIF, which the previous split (async cancel under the lock, sync cancel after release) would have allowed.