ASoC: msm-cpe-lsm: Skip prepare if stream is already prepared

It is possible that userspace may prepare the stream multiple times
without closing the stream. In such cases, preparing the CPE multiple
times can result into an error and eventually leading the userspace to
fail the stream setup. Fix this issue by skipping the stream prepare if
the stream is already prepared once.

Change-Id: Ia5cc3494812497dec46ff84a54711f7b7da6dcab
Signed-off-by: Bhalchandra Gajare <gajare@codeaurora.org>