[RFA] HE addition: TB PPDU bandwidth

Trigger-based PPDUs have an additional bandwidth that determines the
spectral mask etc., I forgot those before but now we need to be able to
look at them in a sniffer.

Since we have a few (very few!) reserved bits, change the HE field as
follows:

data 6

0x00e0  (reserved)

to

0x0020  TB PPDU bandwidth known
0x00c0  TB PPDU bandwidth (bits 24/25 from HE-SIG-A1),
        0: 20 MHz, 1: 40 MHz, 2: 80 MHz, 3: 160 MHz/80+80 MHz


The existing bandwidth field captures only the bandwidth/RU allocation
of the data, which in this case would be the RU allocation for the
captured transmitter, not the bandwidth of the overall transmission(s).

johannes