e1000e: Fix oops caused by ASPM patch.

Commit 6f461f6c7c961f0b1b73c0f27becf472a0ac606b
("e1000e: enable/disable ASPM L0s and L1 and ERT according to hardware errata")
oopses on one of my ppc64 boxes with a NULL pointer (0x4a):

Unable to handle kernel paging request for data at address 0x0000004a
Faulting instruction address: 0xc0000000004d2f1c
cpu 0xe: Vector: 300 (Data Access) at [c000000bec1833a0]
    pc: c0000000004d2f1c: .e1000e_disable_aspm+0xe0/0x150
    lr: c0000000004d2f0c: .e1000e_disable_aspm+0xd0/0x150
   dar: 4a

[c000000bec1836d0] c00000000069b9d8 .e1000_probe+0x84/0xe8c
[c000000bec1837b0] c000000000386d90 .local_pci_probe+0x4c/0x68
[c000000bec183840] c0000000003872ac .pci_device_probe+0xfc/0x148
[c000000bec183900] c000000000409e8c .driver_probe_device+0xe4/0x1d0
[c000000bec1839a0] c00000000040a024 .__driver_attach+0xac/0xf4
[c000000bec183a40] c000000000409124 .bus_for_each_dev+0x9c/0x10c
[c000000bec183b00] c000000000409c1c .driver_attach+0x40/0x60
[c000000bec183b90] c0000000004085dc .bus_add_driver+0x150/0x328
[c000000bec183c40] c00000000040a58c .driver_register+0x100/0x1c4
[c000000bec183cf0] c00000000038764c .__pci_register_driver+0x78/0x128

Seems like pdev->bus->self == NULL. I haven't touched pci in a long time
so I'm trying to remember what this means (no pcie bridge perhaps?)

The patch below fixes the oops for me.

Signed-off-by: Anton Blanchard <anton@samba.org>
Reviewed-by: Bruce Allan <bruce.w.allan@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/e1000e/netdev.c

@@ -4633,6 +4633,9 @@ static void __e1000e_disable_aspm(struct pci_dev *pdev, u16 state)
 	reg16 &= ~state;
 	pci_write_config_word(pdev, pos + PCI_EXP_LNKCTL, reg16);
 
+	if (!pdev->bus->self)
+		return;
+
 	pos = pci_pcie_cap(pdev->bus->self);
 	pci_read_config_word(pdev->bus->self, pos + PCI_EXP_LNKCTL, &reg16);
 	reg16 &= ~state;