usbip: usbip_host_common: Use new error codes to return request status

Currently ST_OK and ST_NA are the only values used to communicate
status of a request from a client. Use new error codes to clearly
indicate what failed. For example, when client sends request to
import a device that isn't export-able, send ST_DEV_BUSY to the client.

Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>