ixgbe: rename ixgbe_ptp_enable to ixgbe_ptp_feature_enable

Since the name ixgbe_ptp_enable could be misconstrued as a function
which enables the whole PTP core, rename this function so that it is
clear the function is for enabling of the extra features such as PPS
signal.

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>