mac80211: enhance readability of Minstrels rc_stats output

This patch restructures the rc_stats debugfs table of Minstrel 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:

    rate      tpt  eprob *prob ret  *ok(*cum)        ok(      cum)
 DP 1          0.9  93.5 100.0   1    0(   0)         2(        2)
    2          0.4  40.0 100.0   0    0(   0)         4(        10)
    5.5        0.0   0.0   0.0   0    0(   0)         0(        0)
...

is changed into this new layout:

best   _______rate_____    __statistics__    ________last_______    ______sum-of________
rate  [name idx tx-time]  [ ø(tp) ø(prob)]  [prob.|retry|suc|att]  [#success | #attempts]
 DP   1     0     9738      0.9    93.5     100.0   1     1 1             2   2
      2     1     4922      0.4    40.0     100.0   1     0 0             4   10
      5.5   2     1858      0.0     0.0       0.0   2     0 0             0   0
...

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>