mt76: add utility functions for deferring work to a kernel thread

In order to avoid keeping work like tx scheduling pinned to the CPU it was
scheduled from, it makes sense to switch from tasklets to kernel threads.

Unlike a workqueue, this one only allows one fixed worker function to be
executed by the worker thread. Because of that, there is less locking
and less code for scheduling involved.
This is important because the tx worker is scheduled often in a hot path

Signed-off-by: Felix Fietkau <nbd@nbd.name>