msm_mpdecision: use own workqueues

This will alter mpdecision to alloc it's own wq rather than using
the predefined queue which is a shared resource.

The WQ is not bound to the issuer CPU (=high-priority), has a
rescue worker task and is freezable.
Also destroy the workqueue on exit, and rename msm_mpdec to
msm_mpdec_init.

Signed-off-by: Dennis Rassmann <showp1984@gmail.com>