net: dsa: slave: chip data is optional, don't dereference NULL

The new binding does not make use of dsa_chip_data, a.k.a cd.  When
retrieving the size of the EEPROM attached to a switch, don't assume
there is a cd attached to the switch structure.

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