ixgbe: (PTP) Fix PPS interrupt code

Driver was enabling PPS interrupt even when user wasn't enabling it via the
ptp core. This patch fixes the PPS so that it is only enabled explicitly, and
moves the interrupt enabling code into the correct location in the driver

Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Cc: Stable <stable@vger.kernel.org> [3.5]
Tested-by: Phil Schmitt <phillip.j.schmitt@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>