flash/nor/stm32l4: Fix stm32l4_probe()

flash/nor/stm32l4: Fix stm32l4_probe()

The current implementation fails due to the assert() statements in
get_stm32l4_rev_str() and get_stm32l4_bank_type_str(). Rearrange the
code in order to fix the problem.

Change-Id: If19c648dec8ddd3ef2fb801150114104b34c3bf2
Signed-off-by: Marc Schink <dev@zapb.de>
Fixes: 64c2e03b23 ("flash/nor: improved API of flash_driver.info & fixed buffer overruns")
Reviewed-on: http://openocd.zylin.com/6326
Tested-by: jenkins
Reviewed-by: Jan Matyas <matyas@codasip.com>
Reviewed-by: Tarek BOCHKATI <tarek.bouchkati@gmail.com>
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>