[ofono] Make DTMF sending via rilmodem more robust

Loop DTMF requests via core, not via driver. Drop unused variables.

Signed-off-by: Martti Piirainen <martti.piirainen@oss.tieto.com>
Merge pull request #102 from marttipiirainen/dtmf_timing_fix

[ofono] Make DTMF sending via rilmodem more robust