rpmsg: qcom_smd: Return error if receive callback is not present

There may be a rx packet on channel while receive callback is
not yet registered. In that case rx packet will be missed and
not delivered to client.

Add check for receive callback. If it is not registered return
-EAGAIN, so that this packet can be retried on next interrupt.

Change-Id: I3371a9faaefe6aa54d0ba4ea5427428932259191
Signed-off-by: Vivek Golani <vgolani@codeaurora.org>