net: ethernet: ti: am65-cpsw-qos: fix dma per-queue rate checks

There is a HW limitation that dma per-queue rate limiting has to be enabled
sequentially from hi->lo channels. The driver performs checks for above,
but does it incorrectly, as result it's not possible to disable rate
limiting for the channel:

echo 100 > /sys/class/net/eth0/queues/tx-7/tx_maxrate
echo 0 > /sys/class/net/eth0/queues/tx-7/tx_maxrate
^ will fail.

This patch fixes above issue.

Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
Signed-off-by: Vignesh Raghavendra <vigneshr@ti.com>