wifi: mt76: mt7915: rework eeprom tx paths and streams init

Rework tx paths and streams init part to improve readability, and make
sure that the available tx streams should be smaller than or equal to
the available tx paths.

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