MIPS: pci-ar724x: setup command register of the PCI controller

The command register of the PCI controller is
not initialized correctly by the bootloader on
some boards and this leads to non working PCI
bus.

Add code to initialize the command register
from the Linux code to avoid this.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Patchwork: http://patchwork.linux-mips.org/patch/4916/
Signed-off-by: John Crispin <blogic@openwrt.org>