rpmsg: qcom_smd: Fix fallback to qcom,ipc parse

rpmsg: qcom_smd: Fix fallback to qcom,ipc parse

mbox_request_channel() returning value was changed in case of error.
It uses returning value of of_parse_phandle_with_args().
It is returning with -ENOENT instead of -ENODEV when no mboxes property
exists.

Fixes: 24fdd5074b20 ("mailbox: use error ret code of of_parse_phandle_with_args()")
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>
Reviewed-by: Stephan Gerhold <stephan.gerhold@linaro.org>
Tested-by: Stephan Gerhold <stephan.gerhold@linaro.org> # msm8939
Signed-off-by: Barnabás Czémán <barnabas.czeman@mainlining.org>
Link: https://lore.kernel.org/r/20250725-fix-qcom-smd-v2-1-e4e43613f874@mainlining.org
Signed-off-by: Bjorn Andersson <andersson@kernel.org>