ath10k: htt: remove some dead code

We added an unnecessary condition here in commit a904417f

 ("ath10k:
add extended per sta tx statistics support").  "legacy_rate_idx" is a u8
so it can't be negative.  The caller doesn't pass negatives either.  I
have deleted this code.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>