do_smhload: fix return code

do_smhload was using a ulong to store the return value from
smh_load_file. That returns an int, where -1 indicates an error. As a
ulong will never be negative, smh_load_file errors were not detected and
so_smhload always returned zero.

Also, when errors were spotted, do_smhload was returning 1, rather than
the enumeration CMD_RET_FAILURE (which is also 1).

Signed-off-by: Ryan Harkin <ryan.harkin@linaro.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>