igc: Change type of the 'igc_check_downshift' method

The 'igc_check_downshift' method always returns 0; there is no need
for a return value so change the type of this method to void.

Signed-off-by: Sasha Neftin <sasha.neftin@intel.com>
Tested-by: Naama Meir <naamax.meir@linux.intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>