msm: gsi: Avoid ring event ring door bell for empty ieob

Avoid ring event ring door bell, if it is empty ieob.
Empty ieob means, the software read pointer is the same
as hardware, no event ring entry process then. In this case,
no need to ring door bell.

Change-Id: I7af7aa2bf0c8a6cda8c7083a1de3c2b80e8ae9b7
Signed-off-by: Bojun Pan <bojunp@codeaurora.org>