mt76: testmode: add support for LTF and GI combinations for HE mode

Prepare for setting different LTF (1x, 2x, and 4x) and GI (0.8us,
1.6us, 3.2us) combinations in HE rate mode.

Reviewed-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Shayne Chen <shayne.chen@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>