rpmsg: glink: Add TX_DATA_CONT command while sending

With current design the transport can send packets of size upto
FIFO_SIZE which is 16k and return failure for all packets above 16k.

Add TX_DATA_CONT command to send packets greater than 16k by splitting
into 8K chunks.

Signed-off-by: Arun Kumar Neelakantam <aneela@codeaurora.org>
Signed-off-by: Deepak Kumar Singh <deesin@codeaurora.org>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Link: https://lore.kernel.org/r/1596086296-28529-4-git-send-email-deesin@codeaurora.org