net: fix kernel-doc warning in <linux/netdevice.h>

Fix missing '*' kernel-doc notation that causes this warning:

../include/linux/netdevice.h:1779: warning: bad line:                                 spinlock

Fixes: ab92d68f

 ("net: core: add generic lockdep keys")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Taehee Yoo <ap420073@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>