sim: bfin: fix sign extension in dsp insns with MM flag

sim: bfin: fix sign extension in dsp insns with MM flag

After testing the hardware with all the different dsp flags, the MM flag
triggers sign extension in all modes.  So drop the limited use of it, and
the local custom helper that was also extending unsigned values.  We also
can see that the flag checks in the mult/mac insns have the same behavior
with sign extending, so add a helper func to keep the logic the same in
both places.

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