firmware_class: Avoid pending list corruption

Remove firmware buffer from pending list when it is freed.
Once the buffer is free kmalloc can allocate the same slab
object for some other user but as the buffer is still there
in the pending list, we end up with multiple users of the
same slab object using it in different contexts.

Change-Id: Id83afbd4416a82646f46b5015d983b9c63a2a613
Signed-off-by: Imran Khan <kimran@codeaurora.org>
Signed-off-by: Rishabh Bhatnagar <rishabhb@codeaurora.org>