mei: make return values consistent across the driver

Follow-up for bits missed in
commit 7ca96aa2
mei: make return values consistent across the driver

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