flash/stm32l4x: zero init stm32l4_flash_bank struct on flash bank initialization

flash/stm32l4x: zero init stm32l4_flash_bank struct on flash bank initialization

This is specially needed when, in the probe routine, device is not
found among the stm32l4_parts. In this case, the stm32l4_flash_bank->part_info
is undefined and inadvertentanly used afterwards:
part_info = stm32l4_info->part_info;

If the stm32l4_flash_bank is zero init, the probe routine checks for
the validity of the part_info field in the previous struct and correctly
detects the unsupported (or not found) condition, raising an error
rather than a SIGSEGV

Change-Id: I7d9d669fb3fa7f8f0903acd60046966b4acb0031
Signed-off-by: Raúl Sánchez Siles <rasasi78@gmail.com>
Reviewed-on: http://openocd.zylin.com/6103
Tested-by: jenkins
Reviewed-by: Tarek BOCHKATI <tarek.bouchkati@gmail.com>
Reviewed-by: Oleksij Rempel <linux@rempel-privat.de>
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>