Most callers of config read do not check for return value. But most of the
ones that do, checks for error indication in 'val' variable.

This patch updates error handling in advk_pcie_rd_conf() function. If PIO
transfer fails then 'val' variable is set to 0xffffffff which indicates
failture.

Link: https://lore.kernel.org/r/20200528162604.GA323482@bjorn-Precision-5520
Link: https://lore.kernel.org/r/20200601130315.18895-1-pali@kernel.orgReported-by: NBjorn Helgaas <helgaas@kernel.org>
Signed-off-by: NPali Rohár <pali@kernel.org>
Signed-off-by: NLorenzo Pieralisi <lorenzo.pieralisi@arm.com>