mtd: st_spi_fsm: Simplify error checking in .probe() a bit

Instead of ending each if branch with the same check, do it once
unconditionally after the if block.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20220607152458.232847-5-u.kleine-koenig@pengutronix.de