Do not run ieee80211_queue_delayed_work for mac work if the worqueue is
not initialized yet

Fixes: b1bd7bb8 ("mt76: connac: unschedule mac_work before going to sleep")
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/5bfd36a75799c37b12fcb54d17e77fbc2c7a0558.1619187875.git.lorenzo@kernel.org