wifi: libertas: fix memory leak in helper_firmware_cb()

In the Linux kernel, the following vulnerability has been resolved: wifi: libertas: fix memory leak in helper_firmware_cb() helper_firmware_cb() neglects to free the single-stage firmware image after a successful async load, leading to a memory leak in the USB firmware-download path. Fix this memory leak by calling release_firmware() immediately after lbs_fw_loaded() returns. The bug was first flagged by an experimental analysis tool we are developing for kernel memory-management bugs while analyzing v6.13-rc1. The tool is still under development and is not yet publicly available. Manual inspection confirms that the bug is still present in the current wireless tree. An x86_64 allyesconfig build showed no new warnings. As we do not have compatible Libertas USB hardware for exercising this firmware-download path, no runtime testing was able to be performed.