flash/nor/stm32h7x: check OPTCHANGEERR

flash/nor/stm32h7x: check OPTCHANGEERR

Without this, a failed attempt to change option bytes will silently
appear to succeed but without actually changing the option bytes
(confusingly, the option bytes will still read back as if they had been
changed until a reboot as well!).

Change-Id: Id529c6c384a8a16be75f5702310670d99d8fac79
Signed-off-by: Christopher Head <chead@zaber.com>
Reviewed-on: http://openocd.zylin.com/5418
Tested-by: jenkins
Reviewed-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>