mac80211: minstrel_ht: remove extraneous indentation on if statement

The increment of idx is indented one level too deeply, clean up the
code by removing the extraneous tab.
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Link: https://lore.kernel.org/r/20210416095137.2033469-1-colin.king@canonical.comSigned-off-by: Johannes Berg <johannes.berg@intel.com>