drivers: flash: stm32h7x: Fix wrong flash write offset

The flash_stm32_write_range() function of the STM32H7x flash
driver partially uses a wrong flash program word size for certain
SOC types when calculating the flash write offset. If the used
SOC is not having a flash program word size of 256 bits / 32 bytes
the written data might get corrupted, as the flash write offset
value does not match the number of bytes that were actually
written.

Fixes #45568

Signed-off-by: Christoph Heller <chris@metanetics.de>