mt76: mt7603: fix tx status HT rate validation

Use the correct variable in the check. Fixes an uninitialized variable warning

Reported-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Fixes: c8846e10 ("mt76: add driver for MT7603E and MT7628/7688")
Signed-off-by: Felix Fietkau <nbd@nbd.name>