rpmsg: glink: Do not send to remote proc in reset

If an intent is available and there is space in the fifo, glink sends
the message and returns success even when the remote proc is in reset.
This causes request/response type drivers to wait for a response from
a remote proc in reset which can delay cleanup. Return an error on send
if the remote proc is in reset.

Change-Id: Ic49aa5e545d871387b8ebd74ad2828ca2140645b
Signed-off-by: Chris Lew <clew@codeaurora.org>