net: ethernet: ti: cpsw: add missing sanity check

Make sure to check for allocation failures before dereferencing a
NULL-pointer during probe.

Fixes: 649a1688 ("net: ethernet: ti: cpsw: create common struct to
hold shared driver data")
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>