kinetis: bugfix in kinetis_write() fallback path

kinetis: bugfix in kinetis_write() fallback path

Offset calculation into buffer was wrong and code would read outside buffer
if count was not a multiple of four.

Change-Id: Ied625b10221423d5a5f25d27ce1edd8c2c3eca8a
Signed-off-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>
Reviewed-on: http://openocd.zylin.com/749
Reviewed-by: Peter Stuge <peter@stuge.se>
Tested-by: jenkins
Reviewed-by: Freddie Chopin <freddie.chopin@gmail.com>