PCI/AER: Clear only ERR_NONFATAL bits during non-fatal recovery

pci_cleanup_aer_uncorrect_error_status() is called by driver .slot_reset()
methods when handling ERR_NONFATAL errors.  Previously this cleared *all*
the bits, including ERR_FATAL bits.

Since we're only handling ERR_NONFATAL errors, clear only the ERR_NONFATAL
error status bits.

Signed-off-by: Oza Pawandeep <poza@codeaurora.org>
[bhelgaas: split to separate patch]
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>