ixgbe: Inline Rx PTP descriptor handling

This change is meant to inline the Rx PTP descriptor handling.  The main
motivation is to avoid unnecessary jumps into function calls that we then
immediately exit because we are not performing timestamps.

The net result of this change is that ixgbe_ptp_rx_tstamp drops from .5% CPU
utilization in my performance runs to 0%, and the only value tested is the Rx
descriptor which should already be warm in the cache if not stored in a
register.

Cc: Jacob Keller <jacob.e.keller@intel.com>
Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Acked-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>