cmd: ubi: Make ubi_detach() static

Its only called from this file, so make it static. While at it, remove
some occurances of multiple blank lines as well.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Boris Brezillon <boris.brezillon@bootlin.com>
Cc: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Boris Brezillon <boris.brezillon@bootlin.com>
Reviewed-by: Jagan Teki <jagan@openedev.com>