i40e/i40evf: fix I40E_MASK signed shift overflow warnings

GCC 6 has a new warning which will display when you attempt to left
shift a signed value beyond the storage size of the type. I40E_MASK
generates a mask value for 32bit registers. Properly typecast the mask
value and place the values in parenthesis to prevent macro expansion
issues.

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