haven: avoid sleeping in atomic context messages

Currently blocking calls are made from atomic context in doorbell and
message queue drivers. Modify the code flow to avoid kernel warnings
when drivers are initializing.

Change-Id: Iae851d5d7e09210a8fb617da0e2cc4cc52c0f258
Signed-off-by: Murali Nalajala <mnalajal@codeaurora.org>