samples: drivers: spi flash testcase check the erased data

After erasing the sector, compare dat read with expected 0xFF pattern
to decide if erasing is successful instead of relying on the
returned code of the flash_erase function

Signed-off-by: Francois Ramu <francois.ramu@st.com>
(cherry picked from commit 325ae4d32a85947c5ee6cef570e8df8c2fa313b0)
1 file changed