fec_restart() runs in softirq context and we should use the
netif_tx_lock_bh/netif_tx_unlock_bh() variants to avoid the following warning
that happens since commit 54309fa6 ("net: fec: fix kernel oops when plug/unplug
cable many times"):

[    9.753168] =================================
[    9.757540] [ INFO: inconsistent lock state ]
[    9.761921] 3.10.0-rc1-next-20130514 #13 Not tainted
[    9.766897] ---------------------------------
[    9.771264] inconsistent {SOFTIRQ-ON-W} -> {IN-SOFTIRQ-W} usage.
[    9.777288] swapper/0 [HC0[0]:SC1[3]:HE1:SE0] takes:
[    9.782261]  (_xmit_ETHER#2){+.?...}, at: [<c03c24a4>] sch_direct_xmit+0xa0/0x2d4
[    9.789879] {SOFTIRQ-ON-W} state was registered at:
[    9.794769]   [<c0059c60>] __lock_acquire+0x528/0x1bc0
[    9.799953]   [<c005b838>] lock_acquire+0xa0/0x108
[    9.804780]   [<c0441320>] _raw_spin_lock+0x28/0x38
[    9.809702]   [<c02f9fc8>] fec_restart+0x5d0/0x664
[    9.814542]   [<c02fa738>] fec_enet_adjust_link+0xa8/0xc0
[    9.819978]   [<c02f7a28>] phy_state_machine+0x2fc/0x370
[    9.825323]   [<c0035ee0>] process_one_work+0x1c0/0x4a0
[    9.830589]   [<c0036594>] worker_thread+0x138/0x394
[    9.835587]   [<c003c620>] kthread+0xa4/0xb0
[    9.839890]   [<c000e820>] ret_from_fork+0x14/0x34
[    9.844728] irq event stamp: 185984
[    9.848226] hardirqs last  enabled at (185984): [<c00232b0>] local_bh_enable_ip+0x84/0xf0
[    9.856450] hardirqs last disabled at (185983): [<c0023270>] local_bh_enable_ip+0x44/0xf0
[    9.864667] softirqs last  enabled at (185966): [<c0023470>] irq_enter+0x64/0x68
[    9.872099] softirqs last disabled at (185967): [<c0023510>] irq_exit+0x9c/0xd8
[    9.879440]
[    9.879440] other info that might help us debug this:
[    9.885981]  Possible unsafe locking scenario:
[    9.885981]
[    9.891912]        CPU0
[    9.894364]        ----
[    9.896814]   lock(_xmit_ETHER#2);
[    9.900259]   <Interrupt>
[    9.902884]     lock(_xmit_ETHER#2);
[    9.906500]
[    9.906500]  *** DEADLOCK ***
Reported-by: Shawn Guo <shawn.guo@linaro.org>
Suggested-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: David S. Miller <davem@davemloft.net>