dsp: Unload the BT modules when APR handle is NULL

As a part of SSR/PDR, afe_close() is called. If APR handle
is NULL, BT modules were not unloaded. Unload the modules
to recover post SSR/PDR.

Change-Id: I69bdb3ab1d55ad9a0a8ed3eed069c97af9143722
Signed-off-by: Harshal Ahire <hahire@codeaurora.org>