drivers: flash: stm32 ospi driver aborts memmap before erase/write

This change is aborting the memoryMapped mode of the octo-flash
before erasing or writing the NOR. Operations are performed in
command mode.
Reading is always performed in MemoryMapped mode (memcopy)

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