mtd: rawnand: stm32_fmc2: manage the get_irq error case

During probe, check the "get_irq" error value.

Signed-off-by: Fabien Dessenne <fabien.dessenne@st.com>
Acked-by: Christophe Kerello <christophe.kerello@st.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>