drivers: flash: STM32WBA flash_stm32wba_fm.c: fix sector erase error

The STM32WBA Flash Manager driver is failing to erase a sector, instead of
erasing one sector, two sectors are erased.
Fix it by correctly calculating the number of sectors to erase

Signed-off-by: Eric Mechin <eric.mechin@st.com>
(cherry picked from commit 62a1b8a3)