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: 53e2822e ("rpmsg: Introduce Qualcomm SMD backend")
Signed-off-by: NBjorn Andersson <bjorn.andersson@linaro.org>

drivers/rpmsg/qcom_smd.c

@@ -739,7 +739,7 @@ static int __qcom_smd_send(struct qcom_smd_channel *channel, const void *data,
 
 	while (qcom_smd_get_tx_avail(channel) < tlen) {
 		if (!wait) {
-			ret = -ENOMEM;
+			ret = -EAGAIN;
 			goto out;
 		}