netpoll: Fix use correct return type for ndo_start_xmit()

[ Upstream commit a54776f2c4939bdee084c9ecd00a4a5a25b7c429 ]

The method ndo_start_xmit() returns a value of type netdev_tx_t. Fix
the ndo function to use the correct type.

Signed-off-by: Yunjian Wang <wangyunjian@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Stable-dep-of: 505ead7ab77f ("netpoll: hold rcu read lock in __netpoll_send_skb()")
Signed-off-by: Sasha Levin <sashal@kernel.org>