flash Kinetis: reduce a flash write message severity to info

There is nothing the user can do if their device does not support sector
programming, there is no reason to have this message at warning level.

Change-Id: Ic9b7386e59b64fece7fbfdc543bdfeeed3eae73d
Signed-off-by: Joakim Nohlgård <joakim.nohlgard@eistec.se>
Reviewed-on: http://openocd.zylin.com/4105


Reviewed-by: Tomas Vanek <vanekt@fbl.cz>
Tested-by: jenkins