PCI: aer-inject: Override PCIe AER Mask Registers

I have several systems which have the same problem:  The PCIe AER
corrected and uncorrected masks have all the error bits set.  This
results in the inablility to test with the aer_inject module & utility
on those systems.

Add the 'aer_mask_override' module parameter which will override the
corrected or uncorrected masks for a PCI device.  The mask will have the
bit corresponding to the status passed into the aer_inject() function.

After this patch it is possible to successfully use the aer_inject
utility on those PCI slots.

Successfully tested by me on a Dell and Intel whitebox which exhibited
the mask problem.

Signed-off-by: Prarit Bhargava <prarit@redhat.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>