last attempted rate,
instead of checking whether ts->ts_rate is set.

Additionally, the conversion from the driver rate index back to the
mac80211 rate index can be dropped, as the mac80211 tx status will still
have the original rate index which was used to set up the descriptor.

Additionally, one more inaccuracy was fixed - the final rate attempt
count only needs to be increased by one if the transmission attempt was
successful.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
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