net: dsa: bcm_sf2: Advertise number of egress queues

The switch supports 8 egress queues per port, so indicate that such that
net/dsa/slave.c::dsa_slave_create can allocate the right number of TX queues.
While at it use SF2_NUM_EGRESS_QUEUE as a define for the number of queues we
support.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>