drivers: flash: fix stm32 ospi prescaler

Original computation seems to be based on
QSPI that has prescaler range [0, 255].
However, OSPI clock prescaler has different
value range [1, 256].

Adapt prescaler computation to valid OSPI
domain and add min value assert.

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