wl18xx: add version 4 of the wl18xx firmware

There are new APIs in the latest version of the wl18xx firmware. Add
the new firmware with a new filename (wl18xx-fw-4.bin), which
corresponds to firmware revision 8.9.0.0.17.

Signed-off-by: Yaniv Machani <yanivma@ti.com>
Signed-off-by: Kyle McMartin <kyle@kernel.org>

WHENCE

@@ -1867,6 +1867,8 @@ File: ti-connectivity/wl18xx-fw-2.bin
 Version: 8.5.0.0.55
 File: ti-connectivity/wl18xx-fw-3.bin
 Version: 8.8.0.0.13
+File: ti-connectivity/wl18xx-fw-4.bin
+Version: 8.9.0.0.17
 File: ti-connectivity/wl18xx-conf.bin
 Version: 6/5