mt76: testmode: add a new state for continuous tx

Support to set a special tx state in testmode: continuous tx,
which is used for sending tx without time gap.
Note that continuous tx mode doesn't send real packets, instead, it's
pure phy signal, and the waveform can be observed by instrument.

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