Motivation for this change

I needed this driver for my new USB WLAN adapter (rtl8811au).

Things done

For this I've merged together

• https://aur.archlinux.org/cgit/aur.git/tree/PKGBUILD?h=rtl8821au-dkms-git
• https://github.com/NixOS/nixpkgs/blob/master/pkgs/os-specific/linux/rtl8812au/default.nix
• https://github.com/NixOS/nixpkgs/blob/master/pkgs/os-specific/linux/rtl8814au/default.nix
  And the result works fine for me.

I guess there's an argument to be made to add a make flag CONFIG_RTL8812A=n, so the resulting module is named "8821a" instead of "8812a" and so doesn't conflict with the better suited driver for this chipset.

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Assured whether relevant documentation is up to date
• Fits CONTRIBUTING.md.