STM32F2x: Don't clear FLASH_OPTCR bits when locking register

STM32F2x: Don't clear FLASH_OPTCR bits when locking register

stm32x_write_options is locking the FLASH_OPTCR register by
writing 0x00000001 to it, which clears the other bits. This
causes problems with subsequent flash operations; the hardware
is probably seeing the write protection bits in the register
set to '0' (protect), causing a WRPERR.
This patch ORs the value of the register with 0x00000001, so that
the only change is the lock bit itself.

Change-Id: I0e3ca9aa6563ce1b57a01fc0faf7563b6b85f620
Signed-off-by: Elliott Partridge <elliott.partridge@gmail.com>
Reviewed-on: http://openocd.zylin.com/2155
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>