net: dsa: bcm_sf2: Remove probing through old DSA binding

Remove our dsa_switch_driver::drv_probe callback to prevent probing
through the old DSA binding, not that this could happen anymore now that
we have moved the matching compatible string from net/dsa/dsa.c to
drivers/net/dsa/bcm_sf2.c, so this is essentially dead code.

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