rpmsg: Fix documentation return formatting

kernel documentation specification:
"The return value, if any, should be described in a dedicated section
named Return."

Signed-off-by: Arnaud Pouliquen <arnaud.pouliquen@foss.st.com>
Link: https://lore.kernel.org/r/20211108140126.3530-1-arnaud.pouliquen@foss.st.com


Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>