igc: Set Qbv start_time and end_time to end_time if not being configured in GCL

The default setting of end_time minus start_time is whole 1 second.
Thus, if it's not being configured in any GCL entry then it will be
staying at original 1 second.

This patch is changing the start_time and end_time to be end_time as
if setting zero will be having weird HW behavior where the gate will
not be fully closed.

Fixes: ec50a9d4

 ("igc: Add support for taprio offloading")
Signed-off-by: Tan Tee Min <tee.min.tan@linux.intel.com>
Signed-off-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>