drivers: flash: stm32g4: Fix erase on wrong bank

In the default configuration of the stm32g4 is the dual bank mode.
In dual bank mode, the flash is split into two banks with 256k each.
The erase pages have a range form 0 to 127, where each page has 2k.
If one wants to erase an area above 256k, the driver has to switch
to bank 2 befor erasing. Otherwise it will erase the page moulo 127.

Signed-off-by: Alexander Wachter <alexander.wacher@leica-geosystems.com>