broadcom-sta: patch for 5.1+

[rembo10]

Motivation for this change

The broadcom-sta package wasn't compiling on kernel 5.1.0. It calls a get_ds() function, which was removed. get_ds() was simply defined as KERNEL_DS, so replacing that function call with KERNEL_DS seems to have fixed things. I pulled the patch from archlinux. Here are some relevant bug reports:
https://bugs.gentoo.org/685214
https://bugs.archlinux.org/task/62604

Things done

• 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 nix-review --run "nix-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.

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[rembo10]

I'll close this in favor of #61686