wifi: mt76: mt7915: cleanup mt7915_get_power_bound

Refactor for making use of mt76_get_power_bound instead of the specific
mt7915_get_power_bound, since we need this for other chipsets as well
when calculating txpower

Signed-off-by: Razvan Grigore <razvan.grigore@vampirebyte.ro>
Link: https://patch.msgid.link/20250211081247.5892-5-razvan.grigore@vampirebyte.ro


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