- Fix calculation of `$vht_cap` bit field
- Replace wrong reference to `$tx_stbc` variable with proper `$tx_stbc_2by1` one
- Emit proper `RX-STBC-{1,12,123,1234}` tokens for the VHT capability list
See https://dev.openwrt.org/ticket/22535 for reference.
Signed-off-by: Scott Shambarger <devel@shambarger.net>