mac80211: remove the need for variable rates_idx

mac80211: remove the need for variable rates_idx

Currently rates_idx is being initialized with the value -1 and this
value is never read so the initialization is redundant and can be
removed. The next time the variable is used it is assigned a value
that is returned a few statements later. Just return i - 1 and
remove the need for rates_idx.

Addresses-Coverity: ("Unused value")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Link: https://lore.kernel.org/r/20200722153830.959010-1-colin.king@canonical.com
Signed-off-by: Johannes Berg <johannes.berg@intel.com>