powerpc/eeh: Use dev_driver_string() instead of struct pci_dev->driver->name

Replace pdev->driver->name by dev_driver_string() for the corresponding
struct device.  This is a step toward removing pci_dev->driver.

Move the function nearer its only user and instead of the ?: operator use a
normal "if" which is more readable.

Link: https://lore.kernel.org/r/20211004125935.2300113-6-u.kleine-koenig@pengutronix.de


Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>