soc: qcom: glink: Check for NULL transmissions

The G-Link core currently assumes that no NULL transmissions will be
sent. However, this is not the case - NULL transmissions are
encountered, but G-Link does not support them.

Modify the core to reject NULL transmissions by returning -EINVAL
when one is encountered.

Change-Id: I9a886cc58b7362e3eb4f3d6cfc7466fa36fe055c
Signed-off-by: Steven Cahail <scahail@codeaurora.org>