mtd: rawnand: stm32_fmc2: use FIELD_PREP/FIELD_GET macros

This patch removes custom macros and uses FIELD_PREP and FIELD_GET macros.
Signed-off-by: Christophe Kerello <christophe.kerello@st.com>
Reviewed-by: Miquel Raynal <miquel.raynal@bootlin.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/1589284068-4079-3-git-send-email-christophe.kerello@st.com