PCI: aardvark: Fix logic in advk_pcie_{rd,wr}_conf()

The PCI configuration space read/write functions were special casing
the situation where PCI_SLOT(devfn) != 0, and returned
PCIBIOS_DEVICE_NOT_FOUND in this case.

However, while this is what is intended for the root bus, it is not
intended for the child busses, as it prevents discovering devices with
PCI_SLOT(x) != 0. Therefore, we return PCIBIOS_DEVICE_NOT_FOUND only
if we're on the root bus.

Fixes: 8c39d710 ("PCI: aardvark: Add Aardvark PCI host controller driver")
Cc: <stable@vger.kernel.org>
Signed-off-by: Victor Gu <xigu@marvell.com>
Reviewed-by: Wilson Ding <dingwei@marvell.com>
Reviewed-by: Nadav Haklai <nadavh@marvell.com>
[Thomas: tweak commit log.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>