MIPS: ath79: fix broken ar724x_pci_{read,write} functions

The current ar724x_pci_{read,write} functions are
broken. Due to that, pci_read_config_byte returns
with bogus values, and pci_write_config_{byte,word}
unconditionally clears the accessed PCI configuration
registers instead of changing the value of them.

The patch fixes the broken functions, thus the PCI
configuration space can be accessed correctly.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/3493/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>