net: lan966x: Add define for number of priority queues NUM_PRIO_QUEUES

Add a define for the number of priority queues on lan966x. Because there
will be more checks for this, so instead of using hardcoded value all
over the place add a define for this.

Signed-off-by: Horatiu Vultur <horatiu.vultur@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>