mac80211: add STBC encoding to ieee80211_parse_tx_radiotap

This patch adds support for STBC encoding to the radiotap tx parse
function. Prior to this change adding the STBC flag to the radiotap
header did not encode frames with STBC.

Signed-off-by: Philipp Borgers <borgers@mi.fu-berlin.de>
Link: https://lore.kernel.org/r/20210125150744.83065-1-borgers@mi.fu-berlin.de
[use u8_get_bits/u32_encode_bits instead of manually shifting]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>