msm: npu: Support delayed unloading NPU fw

In order to reduce the latency caused by loading NPU fw during
switching networks, support delayed unloading fw after network
is unloaded.

Change-Id: Ibb92ed7f55829ac82158792d8630cd5a187de07f
Signed-off-by: Jilai Wang <jilaiw@codeaurora.org>