igc: allow BaseTime 0 enrollment for Qbv

Introduce qbv_enable flag in igc_adapter struct to store the Qbv on/off.
So this allow the BaseTime to enroll with zero value.

Fixes: 61572d5f

 ("igc: Simplify TSN flags handling")
Signed-off-by: Muhammad Husaini Zulkifli <muhammad.husaini.zulkifli@intel.com>
Signed-off-by: Tan Tee Min <tee.min.tan@linux.intel.com>
Tested-by: Naama Meir <naamax.meir@linux.intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>