mac802154: Fix potential RCU dereference issue in mac802154_scan_worker

In the Linux kernel, the following vulnerability has been resolved:

mac802154: Fix potential RCU dereference issue in mac802154_scan_worker

In the `mac802154_scan_worker` function, the `scan_req->type` field was

accessed after the RCU read-side critical section was unlocked. According

to RCU usage rules, this is illegal and can lead to unpredictable

behavior, such as accessing memory that has been updated or causing

use-after-free issues.

This possible bug was identified using a static analysis tool developed

by myself, specifically designed to detect RCU-related issues.

To address this, the `scan_req->type` value is now stored in a local

variable `scan_req_type` while still within the RCU read-side critical

section. The `scan_req_type` is then used after the RCU lock is released,

ensuring that the type value is safely accessed without violating RCU

rules.