Eliminate duplicated code in the handle_mw_command memory write loop.

- wordsize will always be 1, 2, or 4 due to preceeding switch statement.
- move call to keep_alive after successful writes, not upon failures


git-svn-id: svn://svn.berlios.de/openocd/trunk@1953 b42882b7-edfa-0310-969c-e2dbd0fdcd60