The dad_counter is hashed into the resulting address. Since we
want the hashing to be independent of the architecture, we always
hash 32 bit of dad_counter. Make the dad_counter argument of
type guint32 for consistency.

In practice this has no effect because:
  - for all our (current!) architectues, guint is the same as
    guint32.
  - all callers of nm_utils_ipv6_addr_set_stable_privacy() keep
    their dad-counter argument as guint8, so they never even pass
    numbers larger then 255.
  - nm_utils_ipv6_addr_set_stable_privacy() limits dad_counter
    further against RFC7217_IDGEN_RETRIES.