sim: bfin: fix sign extension with 16bit acc add insns

sim: bfin: fix sign extension with 16bit acc add insns

The current implementation attempts to handle the 16bit sign extension
itself.  Unfortunately, it gets it right in some cases.  So rather than
fix that logic, just drop it in favor of using 16bit signed casts.  Now
gcc will take care of getting the logic right.

Signed-off-by: Robin Getz <robin.getz@analog.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>