igb: simplify and clean up igb_enable_mas()

igb_enable_mas() should only be called for the 82575 and has no clear
return so changing it to void. Also simplify the odd conditional
expression.

Signed-off-by: Todd Fujinaka <todd.fujinaka@intel.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>