drivers: flash: flash_stm32g4x.c: take into account bank swap
drivers: flash: flash_stm32g4x.c: take into account bank swap

When erasing flash page, take into account a potential bank swap.

Signed-off-by: Alexandre Bourdiol <c23cc711c078d3e6bb50b3dc45a1b8842691e59a@st.com>

finikorg/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,Vudentz/zephyr,finikorg/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,finikorg/zephyr,galak/zephyr,galak/zephyr,Vudentz/zephyr,Vudentz/zephyr,nashif/zephyr,galak/zephyr,Vudentz/zephyr,nashif/zephyr,Vudentz/zephyr,nashif/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr