drivers: flash: stm32 qspi flash driver valid read or write operations

Like the stm32 flash driver, in case of QSPI nor flash controller
the read or write or erase returns '0' if data is of null size.
This avoids useless QPSI low level access to the controller.

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