flash: fix stm32 flash driver typo's

Change-Id: I37f3fee063d7f8729e057dcfe8904651790439e6
Signed-off-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-on: http://openocd.zylin.com/745


Tested-by: jenkins
Reviewed-by: Freddie Chopin <freddie.chopin@gmail.com>