ipc_service: static_vrings: Set WQ default type to PRIO_COOP

In 92d8329d a new DT property was introduced to set the WQ priority
of the instance. The fallback value when the property was not present
was arbitrarily set to <0 PRIO_PREEMPT>.

The problem is that this value is actually changing the behaviour for
the code that is not explicitly setting the DT property, breaking in
some cases the existing code.

Move the default value to <0 PRIO_COOP> to give the old code a
consistent behaviour before and after the 92d8329d commit.

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