ixgbe: extract PTP clock device from ptp_init

In order to properly handle a suspend/resume cycle, we cannot destroy
the PTP clock device. As part of this, we should only re-create the
device on first initialization. After a resume, when ixgbe_ptp_init is
called, we won't create a new clock, and we will use the old clock
device. To that end, this patch extracts the clock creation out of
ptp_init, and only calls it if we don't already have a ptp_clock
pointer.

Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Tested-by: Phil Schmitt <phillip.j.schmitt@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>