stm32f2x: Fix left shift of negative value

stm32f2x: Fix left shift of negative value

Use unsigned constant for left shift operation in order to avoid the
following error with GCC >= 6.0:

../src/flash/nor/stm32f2x.c: In function ‘stm32x_handle_unlock_command’:
../src/flash/nor/stm32f2x.c:1324:67: error: left shift of negative value [-Werror=shift-negative-value]
   stm32x_info->option_bytes.optcr2_pcrop = OPTCR2_PCROP_RDP | (~1 << bank->num_sectors);

Change-Id: I0ac082bd0dbb8dc2f61ffff8fdf486ab7962d2e0
Signed-off-by: Marc Schink <openocd-dev@marcschink.de>
Reviewed-on: http://openocd.zylin.com/4207
Tested-by: jenkins
Reviewed-by: Andreas Bolsch <hyphen0break@gmail.com>
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-by: Anton Fosselius <anton.fosselius@gmail.com>
Reviewed-by: Esben Haabendal <esbenhaabendal@gmail.com>