net: dsa: bcm_sf2: Use SF2_NUM_EGRESS_QUEUES for CFP

The magic number 8 in 3 locations in bcm_sf2_cfp.c actually designates
the number of switch port egress queues, so use that define instead of
open-coding it.

Reviewed-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>