PCI: Fix uninitialized variable bug in AER injection code

If it was preempted, and the variable aer_mask_override is changed
after the spin_unlock_irqrestore it will write an uninitialized
variable by the pci_write_config_dword() function.

Signed-off-by: Wanlong Gao <wanlong.gao@gmail.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>