steering: suppress ENOENT error messages

If we did not bandsteer a client, failure to remove
the steering files is expected.

chdir(/tmp/wifi/steering/2.4_bf13157e13): No such file or directory
Garbage collecting steering timestamp files failed: No such file or directory

Change-Id: I0b474a367bcf5c2eea38dbce0125c07018375539
1 file changed