media: uvcvideo: Remove unnecessary NULL check before debugfs_remove_recursive

debugfs_remove_recursive() accepts a NULL parameter and returns
immediately, there's no need for a NULL check in the caller.

This issue was detected with the help of Coccinelle.

[Reword commit message, address uvc_debugfs_cleanup_stream()]

Signed-off-by: Gustavo A. R. Silva <garsilva@embeddedor.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>