rpmsg: rpmsg_create_ept() returns NULL on error

The parameter validation incorrectly returned an ERR_PTR(), which is not
handled by the callers to rpmsg_create_ept(), per the definition NULL
should be returned.

Fixes: 93e93244 ("rpmsg: Handle invalid parameters in public API")
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>