mlxsw: pci: Use dev_driver_string() instead of pci_dev->driver->name

mlxsw: pci: Use dev_driver_string() instead of pci_dev->driver->name

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

[bhelgaas: split to separate patch]
Link: https://lore.kernel.org/r/20211004125935.2300113-8-u.kleine-koenig@pengutronix.de
Tested-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>