ASoC: msm: Fixup number of channels for FM transmitter

Backend during FM transmission supports only stereo
channels. Fix up the number of channels to 2 for FM
transmitter.

CRs-fixed: 556019
Change-Id: I98e08432e66e5910edfa5472969e1c638c25ee2c
Signed-off-by: Phani Kumar Uppalapati <phaniu@codeaurora.org>