rpmsg: glink: use put_device() if device_register fail

if device_register() returned an error! Always use put_device()
to give up the reference initialized. And unregister device for
other return error.

Signed-off-by: Arvind Yadav <arvind.yadav.cs@gmail.com>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>