Implement the extended HE-MU info type decoding to show the HE-SIG-B
common contents in the HE-MU radiotap field.

The DW4 data is partially overwritten by the hardware in all cases, so
only the higher 16 bits can be used.  To be able to use it for the HE
SIG-B common data anyway, move the bits around in the following way:

  SIG-B common 0: DW 4 -> DW 7
  SIG-B common 1: DW 7 -> DW 8
  SIG-B common 2: DW 8 -> DW 4 (upper half)
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>