mei: fix return value on disconnection

commit 2d4d5481 upstream.

Correct errno on client disconnection is -ENODEV not -EBUSY

Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: Alexander Usyskin <alexander.usyskin@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>