This was added by mistake in 8f68e26f79ee325669fb7dc7bbd092fccd5b038f when
adding support for G4 MCUs, which does not using this get_bank() function.
FLASH_OPTR_DBANK is only defined on G4 and L4 MCUs, so on H7 this
FLASH_BANK_2 code was being wrongly excluded.
Signed-off-by: Damien George <damien@micropython.org>