igb: use BIT() macro or unsigned prefix

For bitshifts, we should make use of the BIT macro when possible, and
ensure that other bitshifts are marked as unsigned. This helps prevent
signed bitshift errors, and ensures similar style.

Make use of GENMASK and the unsigned postfix where BIT() isn't
appropriate.

Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>