Add leftover filter coefficients in IIR rssi estimation

Fixes: 7bc04215 ("mt76: add driver code for MT76x2e")
Signed-off-by: NLorenzo Bianconi <lorenzo.bianconi@redhat.com>
Acked-by: NFelix Fietkau <nbd@nbd.name>
Signed-off-by: NKalle Valo <kvalo@codeaurora.org>