ath9k: add stability fixes for long standing hang issues (FS#13, #34, #373, #383)

ath9k: add stability fixes for long standing hang issues (FS#13, #34, #373, #383)

The radio would stop communicating completely. This issue was easiest to
trigger on AR913x devices, e.g. the TP-Link TL-WR1043ND, but other
hardware was occasionally affected as well.

The most critical issue was a race condition in disabling/enabling IRQs
between the IRQ handler and the IRQ processing tasklet

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