remoteproc: fix null pointer dereference on glink only platforms

Currently calling list_del on smd subdev remove path results in
null pointer dereference on glink only platforms. Fix this by
adding safety checks in glink/smd subdev remove paths.

Signed-off-by: Sibi Sankar <sibis@codeaurora.org>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>