mt76: mt7915: fix bit fields for HT rate idx

The bit fields of tx rate idx should be 6 bits, otherwise it might be
incorrect in HT mode.
For VHT/HE rates, only 4 bits are actually used by rate idx, the other
2 bits are used for other functions.

Fixes: c31d94af ("mt76: mt7915: fix tx rate related fields in tx descriptor")
Signed-off-by: Shayne Chen <shayne.chen@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>