gianfar: Fix overflow check and return value for gfar_get_cls_all()

This function may currently fill one entry beyond the end of the
array it is given.  It also doesn't return an error code in case
it does detect overflow.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>