igb: Use a 32-bit mask when calculating the flow control watermarks

For some devices, the result of the flow control high watermark gets
truncated when programming it into the registers because of the mask used.
Switch the mask to 32-bit to prevent this from happening.

Signed-off-by: Matthew Vick <matthew.vick@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>