wifi: mt76: mt7915: add spatial extension index support

In previous, we only allow user to configure tx antenna mask
contiguously (e.g. 0x3, 0xf).
This patch allows user to configure tx antenna mask interleavingly
(e.g. 0x5, 0x8). By setting proper antenna mask and nss, user can
prioritized the signal of different antennas, which helps to test
their performance in normal mode.

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