A struct pci_driver is shared across all device instances, so assigning
pci_driver.err_handler once per device isn't really sensible.

Set adf_driver.err_handler statically instead of in adf_enable_aer().
This removes a use of pci_dev->driver, which is a step toward removing
pci_dev->driver altogether.

Since adf_enable_aer() returns zero unconditionally, make it a void
function.

Link: https://lore.kernel.org/r/20211004125935.2300113-10-u.kleine-koenig@pengutronix.deSigned-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>