atl1c: Check return from pci_find_ext_capability() in atl1c_reset_pcie()

The function atl1c_reset_pcie() does not check the return from
pci_find_ext_cabability() where it is getting the postion of the
PCI_EXT_CAP_ID_ERR. It is possible for the return to be 0.

Signed-off-by: Betty Dall <betty.dall@hp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>