wifi: rtw88: Fix __rtw_download_firmware() for RTL8814AU

Don't call ltecoex_read_reg() and ltecoex_reg_write() when the
ltecoex_addr member of struct rtw_chip_info is NULL. The RTL8814AU
doesn't have this feature.

Signed-off-by: Bitterblue Smith <rtl8821cerfe2@gmail.com>
Acked-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Link: https://patch.msgid.link/55b5641f-094e-4f94-9f79-ac053733f2cf@gmail.com