ToneGenerator: Limit tones to 48k

DTMF tones are played at the hardware module sampling rate.
Until now, this rate was limited to 48k. Tones sound not too
pleasant at higher rates (like 96k and 192k). Ergo, limit the rate
to 48k.

Change-Id: I7fbc3185d988628ce3ee478076a322857221dece