PCI: designware: Make config accessor override checking symmetric

Drivers based on the DesignWare core can override the config read accessors
by supplying rd_own_conf() and rd_other_conf() function pointers.
dw_pcie_rd_conf() calls dw_pcie_rd_own_conf() (for accesses to the root
bus) or dw_pcie_rd_other_conf():

  dw_pcie_rd_conf
    dw_pcie_rd_own_conf                # if on root bus
    dw_pcie_rd_other_conf              # if not on root bus

Previously we checked for rd_other_conf() directly in dw_pcie_rd_conf(),
but we checked for rd_own_conf() in dw_pcie_rd_own_conf().

Check for rd_other_conf() in dw_pcie_rd_other_conf() to make this symmetric
with the rd_own_conf() checking, and similarly for the write path.

No functional change intended.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Pratyush Anand <pratyush.anand@gmail.com>