ipc_service: static_vrings: Set WQ priority back to PRIO_PREEMPT

This reverts commit 7f51907f.

The problem with setting the priority at the highest priority possible
is that when the IPC is under high traffic, the WQ could starve the
scheduler.

Move back to a more sane preemptive priority as default value.

Signed-off-by: Carlo Caione <ccaione@baylibre.com>