drivers: flash: stm32f7: Remove useless definition and use CMSIS

Remove useless definiton STM32F7X_SECTOR_MASK and use CMSIS instead.
By the way fix bug as '|' should be in fact '&',
but thanks to '~' inversion, '|' is now good.

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