PCI/DPC: Fix print AER status in DPC event handling

Previously dpc_handler() called aer_get_device_error_info() without
initializing info->severity, so aer_get_device_error_info() relied on
uninitialized data.

Add dpc_get_aer_uncorrect_severity() to read the port's AER status, mask,
and severity registers and set info->severity.

Also, clear the port's AER fatal error status bits.

Fixes: 8aefa9b0 ("PCI/DPC: Print AER status in DPC event handling")
Signed-off-by: Dongdong Liu <liudongdong3@huawei.com>
[bhelgaas: changelog]
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Keith Busch <keith.busch@intel.com>
Cc: stable@vger.kernel.org	# v4.19+