wfx: rename silabs/ into wfx/

WiFi device maintainer expressed he prefers to use the same name for the
driver and the firmware directory[1].

Since the driver is not yet merged, the renaming does not break
anything.

[1]: https://lore.kernel.org/netdev/87sfxlj6s1.fsf@codeaurora.org/

Signed-off-by: Jérôme Pouiller <jerome.pouiller@silabs.com>
Signed-off-by: Josh Boyer <jwboyer@kernel.org>

WHENCE

@@ -5842,10 +5842,10 @@ License: Redistributable. See LICENSE.Lontium for details.
 
 Driver: wfx - Silicon Labs Wi-Fi Transceiver
 
-File: silabs/wfm_wf200_C0.sec
+File: wfx/wfm_wf200_C0.sec
 Version: 3.12.1
 
-Licence: Redistributable. See silabs/LICENCE.wf200 for details.
+Licence: Redistributable. See wfx/LICENCE.wf200 for details.
 Originates from https://github.com/SiliconLabs/wfx-firmware
 
 --------------------------------------------------------------------------