ixgbe: fix X550 PHY function pointers

A subset of the X550 function pointers didn't have default methods. This
didn't cause any issue with previous X550 devices as they were all
redefined.  However future devices will need these default values.

Signed-off-by: Don Skidmore <donald.c.skidmore@intel.com>
Tested-by: Krishneil Singh <krishneil.k.singh@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>