media: uvcvideo: Use indexed loops in uvc_ctrl_init_ctrl()

As shown by the bug introduced in commit 86f7ef77 ("media: uvcvideo:
Add support for per-device control mapping overrides"), the loop style
used by uvc_ctrl_init_ctrl() is error-prone. Rewrite the loops to use
indices instead.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Ricardo Ribalda <ribalda@chromium.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>