mtd: st_spi_fsm: Use the devm_clk_get_enabled() helper function

Use the devm_clk_get_enabled() helper function instead of hand-writing it.
It saves some line of codes.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/4025ec3980a956b0e776024e88ec960afc457501.1681636580.git.christophe.jaillet@wanadoo.fr