mtd: st_spi_fsm: Add a clk_disable_unprepare() in .probe()'s error path

For all but one error path clk_disable_unprepare() is already there. Add
it to the one location where it's missing.

Fixes: 481815a6 ("mtd: st_spi_fsm: Handle clk_prepare_enable/clk_disable_unprepare.")
Fixes: 69d5af8d ("mtd: st_spi_fsm: Obtain and use EMI clock")
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/20220607152458.232847-2-u.kleine-koenig@pengutronix.de