habanalabs: modify the return values of hl_read/write routines

The hl read and write routines implement the hwmon_ops read and write
interface routines respectively.
These routines are expected to return a completion status when called,
which was not the case until this commit.
This commit modifies these routines to return 0 upon success and a
negative error value upon failure.

Signed-off-by: Moti Haimovski <mhaimovski@habana.ai>
Reviewed-by: Oded Gabbay <oded.gabbay@gmail.com>
Signed-off-by: Oded Gabbay <oded.gabbay@gmail.com>