ath10k: htt_rx: fix signedness bug in ath10k_update_per_peer_tx_stats

ath10k: htt_rx: fix signedness bug in ath10k_update_per_peer_tx_stats

Currently, the error handling for the call to function
ath10k_get_legacy_rate_idx() doesn't work because
*rate_idx* is of type u8 (8 bits, unsigned), which
makes it impossible for it to hold a value less
than 0.

Fix this by changing the type of variable *rate_idx*
to s8 (8 bits, signed).

Addresses-Coverity-ID: 1473914 ("Unsigned compared against 0")
Fixes: 0189dbd71cbd ("ath10k: get the legacy rate index to update the txrate table")
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>