Fix receiving USSD messages not part default GSM character set

RIL delivers USSD string already decoded in UTF-8 format. Hence there is
no need for extra encoding/decoding in rilmodem driver and ofono core.
This implementation will bypass the encoding/decoding mechanism by
using hexadecimal value 0xFF as data coding scheme (reserved SMS and CBS
data coding scheme value in 3GPP TS 23.038)

Signed-off-by: Petri Takalokastari <petri.takalokastari@oss.tieto.com>