mtd: rawnand: lpc32xx_mlc: clean the probe function

Before fixing the error path of the probe function, fix the style of the
entire function and particularly the goto labels: they should indicate
what the next cleanup to do is.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>