wifi: mt76: mt7996: hold dev->mt76.mutex while disabling tx worker in SER

In the Linux kernel, the following vulnerability has been resolved:

wifi: mt76: mt7996: hold dev->mt76.mutex while disabling tx worker in SER

mt7996_mac_reset_work() parked the tx worker and disabled the RX/TX NAPIs
before taking dev->mt76.mutex. mt76_worker_disable()/_enable() are plain
kthread park/unpark, not refcounted, and __mt76_set_channel() toggles the
same worker and the MT76_RESET bit under the mutex. An L1 SER racing a
channel switch could therefore have the worker unparked and MT76_RESET
cleared while the reset path resets the DMA rings, corrupting descriptors
or tokens. Take the mutex before disabling the worker, as mt7915 does.