Allow usage for MacBook Air 6.x again

[DanielSiepmann]

I'm on a MacBook 6 using the corresponding files from this project.
Current nixos unstable delivers current kernel LTS 6.x where the mba6x_bl doesn't work with:

building '/nix/store/0hc9jb2sh9897c9y0d1dmlzgjwfqv7wp-etc.drv'...
  CC [M]  /build/source/mba6x_bl.o
/build/source/mba6x_bl.c: In function 'platform_probe':
/build/source/mba6x_bl.c:325:9: error: implicit declaration of function 'acpi_video_set_dmi_backlight_type'; did you mean 'acpi_video_get_backlight_type'? [8;;https://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html#index-Wimplicit-function-declaration-Werror=implicit-function-declaration8;;]
  325 |         acpi_video_set_dmi_backlight_type(acpi_backlight_vendor);
      |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
      |         acpi_video_get_backlight_type
cc1: some warnings being treated as errors
make[2]: *** [/nix/store/gdcczw6d8ycjvnn8hz2k6a8yainkbwsd-linux-6.1.32-dev/lib/modules/6.1.32/source/scripts/Makefile.build:250: /build/source/mba6x_bl.o] Error 1
make[1]: *** [/nix/store/gdcczw6d8ycjvnn8hz2k6a8yainkbwsd-linux-6.1.32-dev/lib/modules/6.1.32/source/Makefile:2012: /build/source] Error 2
make: *** [Makefile:8: all] Error 2
error: builder for '/nix/store/4sw603dvhcfjcvcwqma74rkb6qzcycss-mba6x_bl-unstable-2016-12-08.drv' failed with exit code 2
error: 1 dependencies of derivation '/nix/store/nkbzc0ic61dsl36dmyz60a6m57yg5r3g-kernel-modules.drv' failed to build
error: 1 dependencies of derivation '/nix/store/h25k46fbm3j04f0vj9z885k251n7609n-linux-6.1.32-modules.drv' failed to build
error: 1 dependencies of derivation '/nix/store/cicgd5p9sz2ma956ycz5jw11kncbwp5v-nixos-system-hikari-23.11pre492895.a64b73e07d4.drv' failed to build

I've tried without the module and it looks like everything works if one uses the acpi_video0 as device instead of mba6x_backlight.

I've also opened a PR upstream at Nixpkgs: NixOS/nixpkgs#237574