PCI/AER: Use "aer" variable for capability offset

Previously we used "pos" or "aer_pos" for the offset of the AER Capability.
Use "aer" consistently and initialize it the same way everywhere.  No
functional change intended.

Link: https://lore.kernel.org/r/20200529230915.GA479883@bjorn-Precision-5520


Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com>