Add more details for return code of MI admin cmds

The admin cmd of libnvme-mi returns negitive number(-1) with errno.
Otherwise it returns nvme-mi status byte (byte 4). And status = 0 means
`Success`.

It is the same behavior as the none-mi version. Align the description
of libnvme-mi to libnvme.

Signed-off-by: Hao Jiang <[email protected]>

Author: drakedog2008