sfc: remove ctpio_dmabuf_start from stats

The ctpio_dmabuf_start entry is not actually a stat and shouldn't
be exposed to ethtool.

Fixes: 2c0b6ee8

 ("sfc: expose CTPIO stats on NICs that support them")
Signed-off-by: Bert Kenward <bkenward@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>