sample: subsys: nvs: disco_l475_iot1: use MCU flash instead of QSPI

This allow storage partition to be erased on automatic bench,
(mass erase) and thus test is now passed.
By the way it allows to test MCU flash driver,
whereas other drivers tests operates on QSPI.

Signed-off-by: Alexandre Bourdiol <alexandre.bourdiol@st.com>