wifi: mt76: testmode: use random payload for tx packets

Compared to fixed payload packets, random payload packets have better
measured EVM under the same txpower. Our tests show EVM becomes at least
2-3 dB better in test cases with high rate and long tx length, which also
aligns the testing results to proprietary driver.

Suggested-by: Jm Chen <jm.chen@mediatek.com>
Signed-off-by: Shayne Chen <shayne.chen@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>