drivers: flash: stm32l5: use write-block-size when validating

STM32L5 have a write block size of 8, but STM32U5 and STM32H5 have a
write block size of 16. Use write-block-size from the device tree
instead of hardcoding this value when validating the range of write
operations.

Fixes #60724

Signed-off-by: Florian Vaussard <florian.vaussard@gmail.com>