i40e: break PTP hardware control from ioctl command for timestamp mode

This patch facilitates future work by breaking the PTP hardware control
bits out of the i40e_set_ts_config function. By doing this, we can
maintain state about the 1588 timestamping mode and properly re-enable
to the last known mode during a re-initialize of 1588 bits.

This patch also modifies i40e_ptp_init to call the
i40e_ptp_set_timestamp_mode during the reconfiguration process. A
future patch will ensure that the hwtstamp_config structure is not reset
during this process, so that timestamp mode will be maintained across a
reset.

Change-ID: Ic20832c96c5c512ac203b6c7534e10d891c560f0
Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>