drivers: flash: stm32: qspi: remove flag_quad_io_en field

There is no need in the `flag_quad_io_en` field.
When QE enabling failed, then the driver `init` fails as well.

There is no way to use qspi in quad mode if qe is not enabled.

Signed-off-by: Georgij Cernysiov <geo.cgv@gmail.com>