mac80211: enhance readability of Minstrel-HTs rc_stats output

mac80211: enhance readability of Minstrel-HTs rc_stats output

This patch restructures the rc_stats debugfs table of Minstrel-HT in
order to achieve better human readability. A new layout of the
statistics and a new header is added. In addition to the old layout
there are two new columns of information added:
idx	- representing the rate index of each rate in mac80211 which
	  can be used to set specific rates as fixed rate via debugfs
airtime	- the tx-time in micro seconds that a 1200 Byte packet
	  takes to be transmitted over the air at the given rate

The old layout of rc_stats:

type           rate      tpt eprob *prob ret  *ok(*cum)        ok(      cum)
HT20/LGI       MCS0      5.6 100.0 100.0   1    0(   0)         1(        1)
HT20/LGI   B   MCS1     10.5 100.0 100.0   0    0(   0)         1(        1)
HT20/LGI  A    MCS2     14.8 100.0 100.0   0    0(   0)         1(        1)
...

is changed into this new layout:

            best   ________rate______    __statistics__    ________last_______    ______sum-of________
mode guard #  rate  [name   idx airtime]  [ ø(tp) ø(prob)]  [prob.|retry|suc|att]  [#success | #attempts]
HT20  LGI  1         MCS0     0    1480      0.0      0.0      0.0   1     0 0             0   0
HT20  LGI  1     B   MCS1     1     740     10.5    100.0    100.0   0     0 0             1   1
HT20  LGI  1    A    MCS2     2     496     14.8    100.0    100.0   0     0 0             1   1
...

Signed-off-by: Thomas Huehn <thomas@net.t-labs.tu-berlin.de>
Signed-off-by: Stefan Venz <ikstream86@gmail.com>
Acked-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>