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: Patrice Chotard <patrice.chotard@st.com>