rpmsg: qcom_smd: Add SET signal support

Added support to SET the transport signals by the clients
This fix works for back to back Dial-Up-Network net browsing
without reboot the device.

Change-Id: I9c2bcb65abee9d59acb1739a9144f97dd52a7d35
Signed-off-by: Sivasri Kumar Vanka <sivasri@codeaurora.org>