Restore mwh/mwb functionality.

Half-word and byte writes using mwh/mwb has been completely broken since
bf71e34cbf, because it dispatched all writes, regardless of access size,
to target_write_buffer(), which uses as large accesses as possible.

Partially revert the commit by changing back to the correct write method.

Change-Id: I60731fc576bf4a076a7da02bee7879e121c21d17
Signed-off-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>
Reviewed-on: http://openocd.zylin.com/1628
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>