mei: check for error codes that mei_flow_ctrl_creds retuns

we cannot use if(!mei_flow_ctrl_creds()) logic as
mei_flow_ctrl_creds also negative error codes

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