media: videobuf2-v4l2: remove redundant error test

request_fd is validated under media_request_get_by_fd() just below this
check. Thus remove it.

Suggested-by: Tomasz Figa <tfiga@chromium.org>
Signed-off-by: Helen Koike <helen.koike@collabora.com>
Reviewed-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>