Makefile: Correct the rule removing old of-platdata files

This makes use of makefile variables that don't exist anymore. Fix it and
also remove the object files in that directory.

Also add FORCE as a dependency as required by the if_changed macro.

Fixes 354d2324 ("Makefile: Remove old of-platdata files before regenerating")
Reported-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Signed-off-by: Simon Glass <sjg@chromium.org>