docs: driver-api: bt8xxgpio: Revive dead link

www.bu3sch.de has been unusable for several years, but the same
information is available on bues.ch.
Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Link: https://lore.kernel.org/r/20191004164059.10397-2-j.neuschaefer@gmx.netAcked-by: Michael Büsch <m@bues.ch>
Reviewed-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

Documentation/driver-api/gpio/bt8xxgpio.rst

@@ -2,7 +2,7 @@
 A driver for a selfmade cheap BT8xx based PCI GPIO-card (bt8xxgpio)
 ===================================================================
 
-For advanced documentation, see http://www.bu3sch.de/btgpio.php
+For advanced documentation, see https://bues.ch/cms/unmaintained/btgpio.html
 
 A generic digital 24-port PCI GPIO card can be built out of an ordinary
 Brooktree bt848, bt849, bt878 or bt879 based analog TV tuner card. The