wifi: mt76: add mt76_get_power_bound helper function

This will replace mt7915_get_power_bound function from b/mt7915/mcu.h,
since we will need it also for mt7921 and mt7925

Signed-off-by: Razvan Grigore <razvan.grigore@vampirebyte.ro>
Link: https://patch.msgid.link/20250211081247.5892-2-razvan.grigore@vampirebyte.ro


Signed-off-by: Felix Fietkau <nbd@nbd.name>