soc: qcom: glink_pkt: Check for NULL data before transmitting

G-Link assumes that no NULL data packets will be transmitted, and thus
does not check for such packets. However, some clients attempt to send
such packets, which can cause failures in G-Link.

Detect NULL data packets before transmitting them. Return -EINVAL if one
is encountered.

Change-Id: I54d0797b9f730108228f853e77fffbb7b05f6c89
Signed-off-by: Steven Cahail <scahail@codeaurora.org>