net: mgmt: Use proper coop thread priority value

If user sets CONFIG_NUM_PREEMPT_PRIORITIES=0, then the priority
of the net_mgmt thread will be -1 which is the same as idle thread.
This will trigger assert in kernel as then the minimum coop priority
is -2 in this case. Remove the net_mgmt thread priority setting from
Kconfig file as it is low value and set the coop thread priority
the same way as other network threads are doing it.

Fixes #32375

Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>