mac80211/minstrel: fix !x!=0 confusion

Commit 06d961a8 ("mac80211/minstrel: use the new rate control API")
inverted the condition 'if (msr->sample_limit != 0)' to
'if (!msr->sample_limit != 0)'. But it is confusing both to people and
compilers (gcc5):
net/mac80211/rc80211_minstrel.c: In function 'minstrel_get_rate':
net/mac80211/rc80211_minstrel.c:376:26: warning: logical not is only applied to the left hand side of comparison
   if (!msr->sample_limit != 0)
                          ^

Let there be only 'if (!msr->sample_limit)'.

Fixes: 06d961a8 ("mac80211/minstrel: use the new rate control API")
Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>