Print v4l parser position on error

Print v4l parser position on error

Most parser functions take a **endp argument to indicate the caller
where parsing has stopped. This is currently only used after parsing
something successfully. This patch sets **endp to the erroneous
position in the error case and prints its position after an error
has occured.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>