ixgbe: Assign set_phy_power dynamically where needed

There are various reasons why this method may or may not need to be
defined and some of these we don't know until runtime.  So we will
set the value in get_invariants.

Signed-off-by: Donald C Skidmore <donald.c.skidmore@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>