soc: qcom: glink_bgcom_xprt: Add TX short command

TX short command is used to send data smaller than 16 bytes,
over FIFO instead of AHB.

Added TX short command.

CRs-Fixed: 2304120
Change-Id: Ie632b4aa52b2aec0f4d7e22b41082918faafd251
Signed-off-by: Deepak Kumar Singh <deesin@codeaurora.org>