Enable use of Ieee80211FrequencyBand literals _MHz and _GHz on Windows with MSVC

[abeltrano]

Type

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

Side Effects

• Breaking change
• Non-functional change

Goals

• Allow the _GHz and _MHz literals for Ieee80211FrequencyBand to be used on Windows with MSVC.
• Resolve Allow _GHz operator use on Windows #91.

Technical Details

• Drop the literal definitions from constexpr to const.

Test Results

• All unit tests pass.

Reviewer Focus

• None

Future Work

• It'd be nice to restore this to constexpr.

Checklist

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