docs: isdn: remove hisax references from kernel-parameters.txt

The hisax driver got removed on 85993b8c ("isdn: remove hisax driver"),
but a left-over was kept at kernel-parameters.txt.

Fixes: 85993b8c ("isdn: remove hisax driver")

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>