mtd: st_spi_fsm: Disable clock only after device was unregistered

Until mtd_device_unregister() returns the device is expected to be
operational. So only disable the clock after the mtd is unregistered.

Fixes: 1fefc8ec ("mtd: st_spi_fsm: add missing clk_disable_unprepare() in stfsm_remove()")
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/20220607152458.232847-4-u.kleine-koenig@pengutronix.de