ieee802154: fix __init functions

Commit 3243acd3
("ieee802154: add __init to lowpan_frags_sysctl_register")

added __init to lowpan_frags_ns_sysctl_register instead of
lowpan_frags_sysctl_register

Suggested-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: Fabian Frederick <fabf@skynet.be>
Signed-off-by: David S. Miller <davem@davemloft.net>