ixgbe: cleanup X540 PHY reset function pointer

The X540 PHY reset pointer isn't currently used which is a good thing as it
wouldn't work as implemented.  On top of that the X540 firmware is written
with the assumption that is does not need to be reset for proper
initialization so it's not needed.  I'm just assigning the pointer at NULL
as the current implementation is rather misleading.

Signed-off-by: Don Skidmore <donald.c.skidmore@intel.com>
Tested-by: Stephen Ko <stephen.s.ko@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>