rpmsg: glink: check channel state before serving intent

Receive callback can be registered even before rpmsg driver probe is
complete. Assuming callback register as channel ready and allowing
remote host to send data is incorrect. In such case for any rx packet
callback may return prematurely thereby dropping the rx packet.

Adding a flag to store channel state when it is ready. Check this flag
before allocating rx intent and allowing data communication to proceed.
It will ensure that client driver is able to receive data correctly,
without any rx packet getting dropped.

CRs-Fixed: 2747216
Change-Id: Ib153cfe5689f31d7417622bf6177e9535fda628a
Signed-off-by: Deepak Kumar Singh <deesin@codeaurora.org>