mt76: mt7921s: fix cmd timeout in throughput test

During heavy loading throughtput test, the RX buffer (128)
would be exhausted and some RX pkts dropped randomly. Increase
buffer size from 128 to 512 (a safer length) to avoid tput degrade
or cmd-timeout problem.

Signed-off-by: Deren Wu <deren.wu@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>