ixgbe: cleanup sparse warnings

Sparse complains valid conversions between restricted types, force
attribute is used to avoid those warnings.

Signed-off-by: Cathy Zhou <cathy.zhou@oracle.com>
Reviewed-by: Shannon Nelson <shannon.nelson@oracle.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>