rpmsg: glink: Fix rpmsg_register_device err handling

The device release function is set before registering with rpmsg. If
rpmsg registration fails, the framework will call device_put(), which
invokes the release function. The channel create logic does not need to
free rpdev if rpmsg_register_device() fails and release is called.

Fixes: b4f8e52b

 ("rpmsg: Introduce Qualcomm RPM glink driver")
Cc: stable@vger.kernel.org
Tested-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Signed-off-by: Chris Lew <clew@codeaurora.org>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>