drivers: flash: stm32h7: fix clearing of erased sector number

This fixes the logic in erase_sector() for clearing the previous
sector number. The logic was reversed, clearing bits it should not
clear and keeping previous bits of the sector number. In practice this
does not seem to have had any effect on the current functionality, but
it will start to matter later if e.g. flash interrupts are enabled.

Signed-off-by: Göran Weinholt <goran.weinholt@endian.se>