wireless: T6425: Fixing VHT beamforming for 802.11ac (backport #3576) (#3849)

• wireless: T6425: Fix broken VHT beamforming

(cherry picked from commit f75f0f9c94472f46e056808c3ac6aba809c090f0)

• wireless: T6425: Add smoketests for VHT beamforming

(cherry picked from commit 578fbe0eb436697132e5a738fec5a4ac61ced8da)

• wireless: T6425: adjust to latest country-code changes

Commit 9e22ab6b2a ("wireless: T6318: move country-code to a system wide
configuration") removed the per wifi interface setting for a country-code. This
commit adjust the smoketests to the new design.

(cherry picked from commit 312273c9569d973c510d871adb941709804d8868)

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Co-authored-by: Alain Lamar <alain_lamar@yahoo.de>
Co-authored-by: Christian Breunig <christian@breunig.cc>