msm: mhi_dev: Handle wrap-around case for event ring updates

Asynchronous reads/writes may result in multiple elements being
added to the event ring in a single update. This change accounts
for the wrap-around case when updating a ring with multiple elements.

Change-Id: I4fca542da488ae812dbb57f2d651a472092b39eb
Signed-off-by: Siva Kumar Akkireddi <sivaa@codeaurora.org>