tspp2: Fix event callback deadlock

TSPP2 driver triggers client callback for events.
During the callback tspp2 driver hold the device mutex, which
can cause a deadlock if client calls any TSPP2 driver API
which try to lock this mutex as well.

Change-Id: I9868eb346cb6d332010f5e5bf466adbc537f910f
Signed-off-by: Gilad Broner <gbroner@codeaurora.org>