firmware: qcom: scm: Fix __scm and waitq completion variable initialization

It is possible qcom_scm_is_available() gives wrong indication that
if __scm is initialized while __scm->dev is not and similar issue
is also possible with __scm->waitq_comp.

Fix this appropriately by the use of release barrier and read barrier
that will make sure if __scm is initialized so, is all of its field
variable.

Fixes: d0f6fa7b ("firmware: qcom: scm: Convert SCM to platform driver")
Fixes: 6bf32599

 ("firmware: qcom: scm: Add wait-queue handling logic")
Signed-off-by: Mukesh Ojha <quic_mojha@quicinc.com>
Link: https://lore.kernel.org/r/1711034642-22860-4-git-send-email-quic_mojha@quicinc.com
Signed-off-by: Bjorn Andersson <andersson@kernel.org>