power: make ethdev power management thread unsafe

power: make ethdev power management thread unsafe

Currently, we expect that only one callback can be active at any given
moment, for a particular queue configuration, which is relatively easy
to implement in a thread-safe way. However, we're about to add support
for multiple queues per lcore, which will greatly increase the
possibility of various race conditions.

We could have used something like an RCU for this use case, but absent
of a pressing need for thread safety we'll go the easy way and just
mandate that the API's are to be called when all affected ports are
stopped, and document this limitation. This greatly simplifies the
`rte_power_monitor`-related code.

Signed-off-by: Anatoly Burakov <anatoly.burakov@intel.com>
Acked-by: Konstantin Ananyev <konstantin.ananyev@intel.com>
Tested-by: David Hunt <david.hunt@intel.com>