ixgbe: extract the hardware setup from the ixgbe_ptp_set_ts_config

ixgbe: extract the hardware setup from the ixgbe_ptp_set_ts_config

Currently all of the hardware setup logic for the PTP hardware bits is
buried inside of the ioctl which sets the timestamp configuration. This
makes it hard to use this logic in other places (primarily reset), and
this means we can't restore current timestamp mode upon a MAC reset.
Extracting this logic into a separate function will enable future work
for the ixgbe_ptp_reset function.

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>