net: mvneta: Allow having more than one queue per TC

The current mqprio implementation assumed that we are only using one
queue per TC. Use the offset and count parameters to allow using
multiple queues per TC. In that case, the controller will use a standard
round-robin algorithm to pick queues assigned to the same TC, with the
same priority.

This only applies to VLAN priorities in ingress traffic, each TC
corresponding to a vlan priority.

Signed-off-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Signed-off-by: David S. Miller <davem@davemloft.net>