mt76: testmode: add support to set user-defined spe index

Add spatial extension (spe) index as a configurable parameter in testmode.
This is used for specifically configuring TX path, such as different
WF TX priority, number of antennas and spatial streams.

If spe_idx is not set, TX path depends on tx_antenna_mask; otherwise,
both spe_idx and tx_antenna_mask are referenced to decide TX path.

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