lib: cmsis_rtos_v2: Default thread prioity

If an unitialized/zeroed optional attribute was passed to osThreadNew
the priority would be osThreadNone i.e. uninitialized. This causes an
ASSERT to be hit as the priority isn't valid (it is not between
osPriorityIdle and osPriorityISR).

The fix checks the passed in priority is not osPriorityNone and assigns
osPriorityNormal. This is the correct CMSIS behaviour.

The ASSERT will still be hit if the priority is invalid (<0).

Signed-off-by: Carlos Stuart <carlosstuart1970@gmail.com>