mac80211: minstrel_ht: add basic support for VHT rates <= 3SS@80MHz

mac80211: minstrel_ht: add basic support for VHT rates <= 3SS@80MHz

When the new CONFIG_MAC80211_RC_MINSTREL_VHT is not set (default 'N'),
there is no behavioral change including in sampling and MCS_GROUP_RATES
remains 8.
Otherwise MCS_GROUP_RATES is 10, and a module parameter *vht_only*
(default 'true'), restricts the rates selection to VHT when VHT is
supported.

Regarding the debugfs stats buffer:
It is explicitly increased from 8k to 32k to fit every rates incl. when
both HT and VHT rates are enabled, as for the format, before:
type           rate     tpt eprob *prob ret  *ok(*cum)        ok(      cum)
HT20/LGI ABCDP MCS0     0.0   0.0   0.0   1    0(   0)         0(        0)
after:
 type           rate      tpt eprob *prob ret  *ok(*cum)        ok(      cum)
 HT20/LGI ABCDP MCS0      0.0   0.0   0.0   1    0(   0)         0(        0)
VHT40/LGI       MCS5/2    0.0   0.0   0.0   0    0(   0)         0(        0)

Signed-off-by: Karl Beldan <karl.beldan@rivierawaves.com>
Cc: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>