ipv6: addrconf: silence sparse endianness warnings

Avoid the following sparse __CHECK_ENDIAN__ warnings:

 include/net/addrconf.h:318:25: warning: restricted __be64 degrades to integer
 include/net/addrconf.h:318:70: warning: restricted __be64 degrades to integer
 include/net/addrconf.h:330:25: warning: restricted __be64 degrades to integer
 include/net/addrconf.h:330:70: warning: restricted __be64 degrades to integer
 include/net/addrconf.h:347:25: warning: restricted __be64 degrades to integer
 include/net/addrconf.h:348:26: warning: restricted __be64 degrades to integer
 include/net/addrconf.h:349:18: warning: restricted __be64 degrades to integer

The warnings are false but they make it harder to spot real
bugs.

Signed-off-by: Bjørn Mork <bjorn@mork.no>
Signed-off-by: David S. Miller <davem@davemloft.net>