mhi: core: Protect DB register when handling DB MODE

Possibility exist when handling a DB MODE event another
core to queue a TRE to same channel.  During that time
CC ctxt WP may get updated, however DB MODE event thread
still be using a stale WP.  Add a lock to synchronize
DB MODE event thread and queue TRE thread.

CRs-Fixed: 1005752
Change-Id: I7f285da8751a867a1c3d651466537368799eb657
Signed-off-by: Sujeev Dias <sdias@codeaurora.org>