rpmsg: qcom_smd: Correct return value for O_NONBLOCK

qcom_smd_send() should return -EAGAIN for non-blocking channels with
insufficient space, so that we can propagate this event to user space.

Fixes: 53e2822e56c7 ("rpmsg: Introduce Qualcomm SMD backend")
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
diff --git a/drivers/rpmsg/qcom_smd.c b/drivers/rpmsg/qcom_smd.c
index 92efa74..d003aa8 100644
--- a/drivers/rpmsg/qcom_smd.c
+++ b/drivers/rpmsg/qcom_smd.c
@@ -739,7 +739,7 @@
 
 	while (qcom_smd_get_tx_avail(channel) < tlen) {
 		if (!wait) {
-			ret = -ENOMEM;
+			ret = -EAGAIN;
 			goto out;
 		}