msm: smd: Remove spurious DATA notification

Add a check to make sure that the SMD_EVENT_DATA
notification is only sent if the FIFO head or
tail has been actually updated.

This extra notification is harmless, but adds
interrupt overhead due to the extra unnecessary
processing.

Change-Id: If01b9f1ab7c5df24d4c1ec6e0f2e63cce8a08bb5
Signed-off-by: Eric Holmberg <eholmber@codeaurora.org>