run(4): properly set F_DATAPAD radiotap flag if frame has padding between
frame header and data.

This will fix 'Mysterious OLPC stuff' for received frames and wrong
CCMP / TKIP / data decoding for transmitted frames in net/wireshark
dissector.

While here, drop unneeded comment - net80211 handles padding requirements
for Tx & Rx without driver adjustment.

Tested with D-Link DWA-140 rev B3, STA mode.

MFC after: 1 week