rtw89: Null pointer dereference in rtw89_pci_setup_mapping()

[Bitterblue Smith <rtl8821cerfe2@xxxxxxxxx>]

Hi,

People trying to run Openwrt in Qemu ran into this situation where 
rtwpci->mmap is used before it's initialised:

rtw89_pci_probe()
  rtw89_pci_setup_resource()
    rtw89_pci_setup_mapping()
      rtw89_pci_cfg_dac()
        rtw89_pci_config_byte_set()
          rtw89_pci_read_config_byte()
            rtw89_dbi_read8()
              rtw89_write16()
                rtw89_pci_ops_write16()
                  writew(data, rtwpci->mmap + addr);

Their wifi card is RTL8852BE.

It looks like the problem was introduced in kernel 6.11 by commit
1fd4b3fe52ef ("wifi: rtw89: pci: support 36-bit PCI DMA address").

This is not my bug, I'm just forwarding it from
https://github.com/openwrt/openwrt/issues/17025.