mtd: rawnand: stm32_fmc2: Stop using nand_release()

This helper is not very useful and very often people get confused:
they use nand_release() instead of nand_cleanup().

Let's stop using nand_release() by calling mtd_device_unregister() and
nand_cleanup() directly.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Cc: Christophe Kerello <christophe.kerello@st.com>
Reviewed-by: Christophe Kerello <christophe.kerello@st.com>
Link: https://lore.kernel.org/linux-mtd/20200519130035.1883-53-miquel.raynal@bootlin.com