arm: smh: Use numeric modes for smh_open

There's no point in using string constants for smh_open if we are just
going to have to parse them. Instead, use numeric modes. The user needs
to be a bit careful with these, since they are much closer semantically
to string modes used by fopen(3) than the numeric modes used with
open(2).

Signed-off-by: Sean Anderson <sean.anderson@seco.com>