rpmsg: qcom_smd: Use qcom_smem_is_available()

rpmsg: qcom_smd: Use qcom_smem_is_available()

Rather than looking up a dummy item from SMEM, use the new
qcom_smem_is_available() function to make the code more clear
(and reduce the overhead slightly).

Add the same check to qcom_smd_register_edge() as well to ensure that
it only succeeds if SMEM is already available - if a driver calls the
function and SMEM is not available yet then the initial state will be
read incorrectly and the RPMSG devices might never become available.

Reviewed-by: Konrad Dybcio <konrad.dybcio@linaro.org>
Signed-off-by: Stephan Gerhold <stephan@gerhold.net>
Link: https://lore.kernel.org/r/20230531-rpm-rproc-v3-8-a07dcdefd918@gerhold.net
Signed-off-by: Bjorn Andersson <andersson@kernel.org>