remoteproc: qcom: wcnss: Add iris completion barrier

There is no guarantee that the iris pointer will be assigned before
remoteproc subsystem starts the wcnss rproc, actually it depends how
fast rproc subsystem is able to get the firmware to trigger the start.

This leads to sporadic wifi/bluetooth initialization issue on db410c
with the following output:
 remoteproc remoteproc1: powering up a204000.wcnss
 remoteproc remoteproc1: Booting fw image qcom/msm8916/wcnss.mdt...
 qcom-wcnss-pil a204000.wcnss: no iris registered
 remoteproc remoteproc1: can't start rproc a204000.wcnss: -22

This patch introduces a 'iris_assigned' completion barrier to fix
this issue. Maybe not the most elegant way, but it does the trick.

Signed-off-by: Loic Poulain <loic.poulain@linaro.org>