mtd: rawnand: fsmc: clean the probe function style

Before fixing the error path of the probe function, fix the style of the
probe function and mostly the goto labels: they should indicate what
the next cleanup is, not the point from which they can be accessed.

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