rpmsg: smd: fix memory leak on channel create

Currently a failed allocation of channel->name leads to an
immediate return without freeing channel. Fix this by setting
ret to -ENOMEM and jumping to an exit path that kfree's channel.

Detected by CoverityScan, CID#1473692 ("Resource Leak")

Fixes: 53e2822e ("rpmsg: Introduce Qualcomm SMD backend")
Cc: stable@vger.kernel.org
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>