caif: Fix access to freed pernet memory

unregister_netdevice_notifier() must be called before
unregister_pernet_subsys() to avoid accessing already freed
pernet memory. This fixes the following oops when doing rmmod:

Call Trace:
 [<ffffffffa0f802bd>] caif_device_notify+0x4d/0x5a0 [caif]
 [<ffffffff81552ba9>] unregister_netdevice_notifier+0xb9/0x100
 [<ffffffffa0f86dcc>] caif_device_exit+0x1c/0x250 [caif]
 [<ffffffff810e7734>] sys_delete_module+0x1a4/0x300
 [<ffffffff810da82d>] ? trace_hardirqs_on_caller+0x15d/0x1e0
 [<ffffffff813517de>] ? trace_hardirqs_on_thunk+0x3a/0x3
 [<ffffffff81696bad>] system_call_fastpath+0x1a/0x1f

RIP
 [<ffffffffa0f7f561>] caif_get+0x51/0xb0 [caif]

Signed-off-by: Sjur Brændeland <sjur.brandeland@stericsson.com>
Acked-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: David S. Miller <davem@davemloft.net>