media: uvcvideo: Fix invalid pointer in uvc_ctrl_init_ctrl()

The handling of per-device mappings introduced in commit 86f7ef77
("media: uvcvideo: Add support for per-device control mapping
overrides") overwrote the mapping variable after it was initialized and
before it was used, leading to usage of an invalid pointer for devices
with per-device mappings. Fix it.

Fixes: 86f7ef77 ("media: uvcvideo: Add support for per-device control mapping overrides")
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>