mac80211: cosmetics for minstrel_debugfs

This changes the minstrel stats ouput from:

rate     throughput  ewma prob   this prob  this succ/attempt   success    attempts
 BCD   6         0.0        0.0        0.0          0(  0)          0           0

to:

rate      throughput  ewma prob  this prob  this succ/attempt   success    attempts
 BCD   6         0.0        0.0        0.0             0(  0)         0           0

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