ixgbe: fix PTP ethtool timestamping function

This patch fixes a development issue that occurred due to invalid modes reported
in the ethtool get_ts_info function. The issue is resolved by removing
unsupported modes from the Rx supported list.

CC: stable <stable@vger.kernel.org> [3.5]
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>