igb: specify phc_index of 82575 for get_ts_info

82575 has only software timestamping capability and it has
no PTP Hardware Clocks. Therefore, -1 has to be specified
to the phc_index for ethtool's get_ts_info, otherwise a wrong
value will be set to the phc_index.

v2: move the if (adapter->ptp_clock) section specifying phc_index
    to above the switch statement as suggested by Matthew Vick.
    adapter->ptpclock will always be NULL for 82575.

Signed-off-by: Ken ICHIKAWA <ichikawa.ken@jp.fujitsu.com>
Acked-by: Matthew Vick <matthew.vick@intel.com>
Tested-by: Jeff Pieper <jeffrey.e.pieper@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>