net/sfc: reserve internal Rx queue for counters

net/sfc: reserve internal Rx queue for counters

MAE delivers counters data as special packets via dedicated Rx queue.
Reserve an RxQ so that it does not interfere with ethdev Rx queues.
A routine will be added later to handle these packets.

There is no point to reserve the queue if no service cores are
available and counters cannot be used.

Signed-off-by: Igor Romanov <igor.romanov@oktetlabs.ru>
Signed-off-by: Andrew Rybchenko <andrew.rybchenko@oktetlabs.ru>
Reviewed-by: Andy Moreton <amoreton@xilinx.com>
Reviewed-by: Ivan Malov <ivan.malov@oktetlabs.ru>