tests/drivers/soc_flash_nrf: fix lack of disabled protection

Write protection was not disabled prior erase call.
It used to work before only because nrf driver had not check
the protection before erase execution. This behavior was changed in
order to align to zephyr - which implied requirement for this test fix.

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