mt76: testmode: make tx queued limit adjustable

Originally, tx queued limit is set to 1000 to prevent from running out
of tx token. If a new testmode tx is triggered while the previous one
hasn't finished yet, we'll wait a period of time until tx_done equals to
tx_queued. Normally, current queued limit can finish in 10 seconds.

However, if ipg is configured to a larger value, less than 1000 packets
can be done in the default timeout period, which may lead to a crash
when a new testmode tx triggered.

To deal with this, make tx queued limit dynamically adjusted according
to ipg value.

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