igc: Decrease PTM short interval from 10 us to 1 us

With the 10us interval, we were seeing PTM transactions take around 12us.
Hardware team suggested this interval could be lowered to 1us which was
confirmed with PCIe sniffer. With the 1us interval, PTM dialogs took
around 2us.

Suggested-by: Vinicius Costa Gomes <vinicius.gomes@intel.com>
Signed-off-by: Sasha Neftin <sasha.neftin@intel.com>
Tested-by: Muhammad Husaini Zulkifli <muhammad.husaini.zulkifli@intel.com>
Reviewed-by: Muhammad Husaini Zulkifli <muhammad.husaini.zulkifli@intel.com>
Tested-by: Naama Meir <naamax.meir@linux.intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>