radio: iris: Add the parameter to turn on/off sig blend

FM driver will always enable the sig blend feature, it will cause
FM seperation failure during lab test.So add this new parameter
to disable the feature by adb command to pass the test.

Change-Id: I187d4e5fff4aabfb994a905e174bb8ce941e2c16
Signed-off-by: Sheng Fang <fsheng@codeaurora.org>