mt76: connac: adjust wlan_idx size from u8 to u16

Newer chips such as MT7915 require up to 16-bits for this field.

Fixes: 49126ac1 ("mt76: connac: move mt76_connac_mcu_bss_basic_tlv in connac module")
Signed-off-by: Chad Monroe <chad.monroe@smartrg.com>
Acked-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>