drivers: flash: stm32 qspi driver when Dual-Flash not supported

Some stm32 devices with quadspi (like stm32l47x or stm32l48x)
does not support Dual-Flash Mode. Avoid building error even if
the &quadspi node has a <flash-id>  property defined.

Signed-off-by: Francois Ramu <francois.ramu@st.com>