flash/stm32l4x: fix maybe-uninitialized compiler error

flash/stm32l4x: fix maybe-uninitialized compiler error

using gcc (Ubuntu 9.3.0-17ubuntu1~20.04) 9.3.0 we get:
error: ‘retval’ may be used uninitialized in this function

fixes: 13cd75b6ecfd (flash/nor/stm32xx: fix segfault accessing Cortex-M part number)
Change-Id: I897c40c5d2233f50a5385d251ebfa536023e5cf7
Signed-off-by: Tarek BOCHKATI <tarek.bouchkati@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/6861
Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>