drivers/flash/flash_stm32: integrate WP service into write/erase

Flash write protection services were integrated into erase and write
procedures. This is step required for fixing following issue:
Multi-threading flash access is not supported by
flash_write_protection_set().

flash_write_protection_set() will be deprecated

Signed-off-by: Andrzej Puzdrowski <andrzej.puzdrowski@nordicsemi.no>