firmware: arm_ffa: Fix memory leak by freeing notifier callback node

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

firmware: arm_ffa: Fix memory leak by freeing notifier callback node

Commit e0573444edbf ("firmware: arm_ffa: Add interfaces to request notification callbacks") adds support for notifier callbacks by allocating and inserting a callback node into a hashtable during registration of notifiers. However, during unregistration, the code only removes the node from the hashtable without freeing the associated memory, resulting in a memory leak.

Resolve the memory leak issue by ensuring the allocated notifier callback node is properly freed after it is removed from the hashtable entry.