ipv6 addrconf: revert /proc/net/if_inet6 ifa_flag format

Turned out that applications like ifconfig do not handle the change.
So revert ifa_flag format back to 2-letter hex value.

Introduced by:
commit 479840ff
    "ipv6 addrconf: extend ifa_flags to u32"

Reported-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: Jiri Pirko <jiri@resnulli.us>
Tested-by: FLorent Fourcot <florent.fourcot@enst-bretagne.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>