flash/nor/stm32f1x: fix options reading from locked chip

flash/nor/stm32f1x: fix options reading from locked chip

In commit cea40152f82f52bfc718c7bda9fa4a9d70bf9cfd option bytes
reading was changed to direct access to option bytes area.
While there are no problems with stm32f0xx and stm32f3xx chips,
option block (0x1ffff800..0x1ffff80F) is unreadable from locked
stm32f10x chips.
As a result, stm32f1x unlock command writes dirty values to user
options, user data and write protection bits.

Option bytes reading reverted from direct access to option bytes area
to reading currently loaded bytes from  FLASH_OBR/FLASH_WRPR registers.

Tested on stm32f100, stm32f103, stm32f107 as well as on stm32f030 and
stm32f303.

Change-Id: Iad476351ffdaca5ace12e02272dacea7f3d08f52
Signed-off-by: Oleksandr Redchuk <real@real.kiev.ua>
Reviewed-on: http://openocd.zylin.com/4940
Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>