arm: smh: Return errno on error

arm: smh: Return errno on error

Instead of printing in what are now library functions, try to return a
numeric error code. This also adjust some functions (such as read) to
behave more similarly to read(2). For example, we now return the number
of bytes read instead of failing immediately on a short read.

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