mac80211: switch to upstream owl-loader driver

The Owl Loader (named after the codename that Atheros gave
these devices back in the day) has been accepted upstream.

This patch removes the "misc" driver OpenWrt had and adds
the remaining differences against the version that ships
with 5.4-rc1 into a separate "120-owl-loader-compat.patch"
file that can be cut down once AR71XX is being dealt with.

Note: I decided to keep the existing (kmod-)owl-loader
package name around for now. The kernel module file in
the kmod package will be called ath9k_pci_owl_loader.ko
though.

Acked-by: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: Christian Lamparter <chunkeey@gmail.com>