igb: drop unnecessary constant casts to u16

Remove unnecessary casts of constant values to u16.
C's integer promotion rules make them ints no matter what.

Additionally replace IGB_MNG_VLAN_NONE with resulting value
rather than casting -1 to u16.

Signed-off-by: Jacek Kowalski <jacek@jacekk.info>
Suggested-by: Simon Horman <horms@kernel.org>
Reviewed-by: Aleksandr Loktionov <aleksandr.loktionov@intel.com>
Tested-by: Rinitha S <sx.rinitha@intel.com> (A Contingent worker at Intel)
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>