mtd: st_spi_fsm: add missing clk_disable_unprepare() in stfsm_remove()

Clock source is prepared and enabled by clk_prepare_enable()
in probe function, but not disabled or unprepared in remove
function.

Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20220516092911.953066-1-yangyingliang@huawei.com