i40e: don't store user requested mode until we've validated it

This patch prevents the SIOCGHWTSTAMP ioctl from possibly returning bad
data, by not permanently storing the setting into the private
structure until after we've finished validating that we can support it.

Change-ID: Ib59f9b4f73f451d5a2e76fb8efa5d4271b218433
Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>