gianfar: Remove unused gfar_add_fcb() function parameter

- Also, use cacheable_memzero instead of memset for performance reasons.

Signed-off-by: Dai Haruki <dai.haruki@freescale.com>
Acked-by: Andy Fleming <afleming@freescale.com>
Signed-off-by: David S. Miller <davem@davemloft.net>