CVE-2022-50573 wifi: mt76: mt7915: fix mt7915_rate_txpower_get() resource leaks

In the Linux kernel, the following vulnerability has been resolved: wifi: mt76: mt7915: fix mt7915ratetxpowerget resource leaks Coverity message: variable "buf" going out of scope leaks the storage. Addresses-Coverity-ID: 1527799 "Resource leaks"...