rpmsg: smd: Signal the TX channel that smd driver read data

Signal the TX channel instead of RX channel before sending interrupt
to remote processor to indicate that smd driver read the data in RX
channel.

Change-Id: I76e3cd45a80794f73aba5a18474dad8429d53c72
Signed-off-by: Prudhvi Yarlagadda <pyarlaga@codeaurora.org>