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: ab92d68fc2 ("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>

include/linux/netdevice.h

@@ -1775,7 +1775,7 @@ enum netdev_priv_flags {
  *			for hardware timestamping
  *	@sfp_bus:	attached &struct sfp_bus structure.
  *	@qdisc_tx_busylock_key: lockdep class annotating Qdisc->busylock
-				spinlock
+ *				spinlock
  *	@qdisc_running_key:	lockdep class annotating Qdisc->running seqcount
  *	@qdisc_xmit_lock_key:	lockdep class annotating
  *				netdev_queue->_xmit_lock spinlock