rpmsg: qcom_smd: Make qcom_smd_unregister_edge() return void

This function returned zero unconditionally. Convert it to return no
value instead. This makes it more obvious what happens in the callers.

One caller is converted to return zero explicitly. The only other caller
(smd_subdev_stop() in drivers/remoteproc/qcom_common.c) already ignored
the return value before.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Bjorn Andersson <andersson@kernel.org>
Link: https://lore.kernel.org/r/20230321154039.355098-2-u.kleine-koenig@pengutronix.de