iwlwifi: make bitfield a u32 instead of u16

The bitfield we use in struct iwl_cfg contains 17 bits, but we declare
it as u16.  The compiler doesn't seem to have any problems with that
(it probably automatically makes it u32), but it's cleaner to use a
type that is long enough for all the flags.

Signed-off-by: Luca Coelho <luciano.coelho@intel.com>