iwlwifi: pcie: set RB chunk size back to 64

128 byte chunk size is supported only on PCIe and not
on IOSF. For now, change it back to 64 byte.

Reported-by: Oren Givon <oren.givon@intel.com>
Signed-off-by: Sara Sharon <sara.sharon@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>