ixgbe: separate the PTP suspend and stop actions

Since we are adding proper support for suspend of PTP, extract out of
ixgbe_ptp_stop those things relevant to suspend. Then, have
ixgbe_ptp_stop call ixgbe_ptp_suspend. The next patch in the series will
have ixgbe_ptp_suspend called from the ixgbe_suspend path.

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>