media: v4l2-subdev: remove wrong @cond from kdocs

v4l2_subdev_call() is unconditional, so don't mention in the docs a
@cond parameter which does not exist.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>