sim: bfin: update return value in bfin_reg_{fetch,store}

sim: bfin: update return value in bfin_reg_{fetch,store}

The latest gdb sim-remote.c really wants a return value from the fetch/store
register functions, so update the Blackfin sim to avoid the warnings/errors.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>