ath9k: add option to reset the wifi chip via debugfs

Sometimes, in yet unknown cases the wifi chip stops working. To allow a
watchdog in userspace to easily and quickly reset the wifi chip, add the
according functionality to userspace. A reset can then be triggered
via:

  $ echo 1 > /sys/kernel/debug/ieee80211/phy0/ath9k/reset

The number of user resets can further be tracked in the row "User reset"
in the same file.

So far people usually used "iw scan" to fix ath9k chip hangs from
userspace. Which triggers the ath9k_queue_reset(), too. The reset file
however has the advantage of less overhead, which makes debugging bugs
within ath9k_queue_reset() easier.

Signed-off-by: Linus Lüssing <ll@simonwunderlich.de>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20210914192515.9273-2-linus.luessing@c0d3.blue