ixgbe: Use method pointer to access IOSF devices

Provide method pointers and use them to access IOSF-attached
devices. A new MAC will introduce a new access method.

Signed-off-by: Mark Rustad <mark.d.rustad@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>