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: NMuhammad Husaini Zulkifli <muhammad.husaini.zulkifli@intel.com>
Signed-off-by: NTan Tee Min <tee.min.tan@linux.intel.com>
Tested-by: NNaama Meir <naamax.meir@linux.intel.com>
Signed-off-by: NTony Nguyen <anthony.l.nguyen@intel.com>