rpmsg: unlock on error in rpmsg_eptdev_read()

We should unlock before returning if skb_dequeue() returns a NULL.

Fixes: c0cdc19f ("rpmsg: Driver for user space endpoint interface")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>