mtd: st_spi_fsm: Handle clk_prepare_enable/clk_disable_unprepare.

- clk_prepare_enable() can fail here and we must check its return value.
 - stfsm_probe() can fail here and we must disable clock.

Signed-off-by: Arvind Yadav <arvind.yadav.cs@gmail.com>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>