Fix some issues in the mac80211_hwsim build script

[abeltrano]

Type

• Bug fix
• Feature addition
• Feature update
• Documentation
• Build Infrastructure

Side Effects

• Breaking change
• Non-functional change

Goals

• Ensure the mac80211_hwsim module can be built properly for the docker container and/or with use of the build-mac80211_hwsim-kmod.sh script.

Technical Details

• Make the old default config prior to configuring new modules.
• Only download source if not already done.

Test Results

• Ran the script and verified the mac80211_hwsim module was built and installed.

Reviewer Focus

• None

Future Work

• None

Checklist

• Build target all compiles cleanly.
• clang-format and clang-tidy deltas produced no new output.
• Newly added functions include doxygen-style comment block.