PCI: Fix oops in pci_vpd_truncate

pci_vpd_truncate() should check for dev->vpd->attr, otherwise
this might happen:

sky2 driver version 1.22
Unable to handle kernel paging request for data at address 0x0000000c
Faulting instruction address: 0xc01836fc
Oops: Kernel access of bad area, sig: 11 [#1]
[...]
NIP [c01836fc] pci_vpd_truncate+0x38/0x40
LR [c029be18] sky2_probe+0x14c/0x518
Call Trace:
[ef82bde0] [c029bda4] sky2_probe+0xd8/0x518 (unreliable)
[ef82be20] [c018a11c] local_pci_probe+0x24/0x34
[ef82be30] [c018a14c] pci_call_probe+0x20/0x30
[ef82be50] [c018a330] __pci_device_probe+0x64/0x78
[ef82be60] [c018a44c] pci_device_probe+0x30/0x58
[ef82be80] [c01aa270] really_probe+0x78/0x1a0
[ef82bea0] [c01aa460] __driver_attach+0xa4/0xa8
[ef82bec0] [c01a96ac] bus_for_each_dev+0x60/0x9c
[ef82bef0] [c01aa0b4] driver_attach+0x24/0x34
[ef82bf00] [c01a9e08] bus_add_driver+0x12c/0x1cc
[ef82bf20] [c01aa87c] driver_register+0x6c/0x110
[ef82bf30] [c018a770] __pci_register_driver+0x4c/0x9c
[ef82bf50] [c03782c8] sky2_init_module+0x30/0x40
[ef82bf60] [c0001dbc] do_one_initcall+0x34/0x1a0
[ef82bfd0] [c0362240] do_initcalls+0x38/0x58

This happens with CONFIG_SKY2=y, and "ip=on" kernel command line, so
pci_vpd_truncate() is called before late_initcall(pci_sysfs_init),
therefore ->attr isn't yet initialized.

Acked-by: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: Anton Vorontsov <avorontsov@ru.mvista.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>