iwlwifi: pcie: set RB chunk size per bus

For 9000 devices we can have PCIe bus for discrete
devices and IOSF bus for integrated devices.
PCIe supports maximum transfer size of 128B while IOSF
bus supports maximum transfer size of 64B.
Configure RB size accordingly.

Signed-off-by: Sara Sharon <sara.sharon@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>