mesh: Implement use of VHT20 config in mesh mode

mesh: Implement use of VHT20 config in mesh mode

Mesh in VHT mode is supposed to be able to use any bandwidth that VHT
supports, but there was no way to set VHT20 although there are
parameters that are supposed to be used. This commit along then previous
commit for VHT_CHANWIDTH_USE_HT makes mesh configuration available to
use any bandwidth with combinations of existing parameters like shown
below.

VHT80:
  default
  do not set any parameters
VHT40:
  max_oper_chwidth = 0
VHT20:
  max_oper_chwidth = 0
  disable_ht40 = 1
HT40:
  disable_vht = 1
HT20:
  disable_ht40 = 1
disable HT:
  disable_ht = 1

Signed-off-by: Peter Oh <peter.oh@bowerswilkins.com>