wifi: mt76: mt7915: fix mt7915_rate_txpower_get() resource leaks

Coverity message: variable "buf" going out of scope leaks the storage.

Reported-by: coverity-bot <keescook+coverity-bot@chromium.org>
Addresses-Coverity-ID: 1527799 ("Resource leaks")
Fixes: e3296759

 ("wifi: mt76: mt7915: enable per bandwidth power limit support")
Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>