cmd_mem: call unmap_sysmem() after map_sysmem()

Actually, unmap_sysmem() does nothing.  Just in case.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Cc: Simon Glass <sjg@chromium.org>
Acked-by: Simon Glass <sjg@chromium.org>