board: cssi: Properly initialise MAC address for fibre on CMPC885 board

CMPC885 board can be pluged on a mother board with fibre interface, so
fibre interface MAC address must be initialised for that case.

Signed-off-by: Hugo Dubois <hugo.dubois.ext@csgroup.eu>
Reviewed-by: CASAUBON Jean Michel <jean-michel.casaubon@csgroup.eu>
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>