rpmsg: smd: Fix double unlock in __qcom_smd_send()

We're not holding the lock here, so we shouldn't unlock.

Fixes: 178f3f75bb4e ("rpmsg: smd: Don't hold the tx lock during wait")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
[bjorn: renamed "out" label to further distinguish the two exit paths]
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>