igc: Add set/clear large buffer helpers

While commit 13b5b7fd ("igc: Add support for Tx/Rx rings")
introduced code to handle larger packet buffers, it missed the
set/clear helpers which enable/disable that feature. Introduce
the missing helpers which will be use in the next patch.

Signed-off-by: Andre Guedes <andre.guedes@intel.com>
Signed-off-by: Vedang Patel <vedang.patel@intel.com>
Signed-off-by: Jithu Joseph <jithu.joseph@intel.com>
Reviewed-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Tested-by: Dvora Fuxbrumer <dvorax.fuxbrumer@linux.intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>