Correct address for STM32F2 flash size register

There was a typo in the flash size register address for STM32F2. Change
to correct address. Verified against STM32F207 reference manual
(RM0033 Rev 5 section 33.2), and an STM32F217 chip.
Remove unsupported --device option to st-util

Setting the STLINK device with -d hasn't worked for some time, but
the STLINK_DEVICE environment variable works instead. Remove the
option, update documentation and help text.