ixgbe: function comment typo

Seems the ixgbe's code was copied from e1000.
The comment talks about something not exist.

Signed-off-by: Wang Chen <wangchen@cn.fujitsu.com>
Acked-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>