i40e: PTP - avoid aggregate return warnings

Aggregate return warnings are when struct types are returned
and must be copied to the lvalue with a struct copy by the compiler.

This fixes warnings of type aggregate-return from gcc with W=2.

Change-Id: I896b1bf514544bf0faeb458869d79914b9f1b168
Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>