soc: qcom: glink_smd_xprt: Make tx() atomic

Some client usecases may cause tx() to be called in atomic context, which
leads to a sleeping while atomic warning due to a GFP_KERNEL allocation.
Make the allocation GFP_ATOMIC to silence the warning.

Change-Id: I02a24ab3cd3fd870c8554141583b14a123d80c66
Signed-off-by: Jeffrey Hugo <jhugo@codeaurora.org>