wifi: mt76: mt7921s: fix workqueue problem causes STA association fail

The ieee80211_queue_work function queues work into the mac80211
local->workqueue, which is widely used for mac80211 internal
work processes. In the mt76 driver, both the mt76-sido-status and
mt76-sdio-net threads enqueue workers to the workqueue with this
function. However, in some cases, when two workers are enqueued
to the workqueue almost simultaneously, the second worker may not
be scheduled immediately and may get stuck for a while.
This can cause timing issues. To avoid these timing
conflicts caused by worker scheduling, replace the worker
with an independent thread.

Fixes: 48fab5bb ("mt76: mt7921: introduce mt7921s support")
Signed-off-by: Wang Zhao <wang.zhao@mediatek.com>
Signed-off-by: Deren Wu <deren.wu@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>