rpmsg: char: Remove the unneeded result variable

Return the value rpmsg_chrdev_eptdev_add() directly instead of storing it
in another redundant variable.

Reported-by: Zeal Robot <zealci@zte.com.cn>
Signed-off-by: ye xingchen <ye.xingchen@zte.com.cn>
Link: https://lore.kernel.org/r/20220826071954.252485-1-ye.xingchen@zte.com.cn


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