Fix an erase error in stm32f030 device.

I run into flashing error with stm32f030. With some
debugging I find out that the mass erase bit was dropped
when flash programming bit already set on FLASH_CR.
This cause erase and write error with my stm32f030
board.

Mass erase never need the programming bit. Turn off the
programming bit make stlink flash stm32f030 successfully.