net: dsa: bcm_sf2: Fix build module

Commit 7318166c ("net: dsa: bcm_sf2: Add support for
ethtool::rxnfc") added a new object to build: bcm_sf2_cfp.o, but in
doing so, we essentially just built this object and no longer bcm_sf2.o.

Fix this by creating a module named bcm-sf2.ko which links in bcm_sf2.o
and bcm_sf2_cfp.o.

Fixes: 7318166c ("net: dsa: bcm_sf2: Add support for ethtool::rxnfc")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>