bonding: add more ethtool support

This patch allows reporting the link, checksum, and feature settings
of bonded device by using generic hooks.

Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
Acked-by: Jay Vosburgh <fubar@us.ibm.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>