si4708: update fm driver

This is a combination of 6 commits:

si4708: uncrustify
si4708: poll for successful seek
si4708: fix channel space mask
si4708: add de-emphasis ioctl
si4708: add RDS support
si4708: add mute support

Change-Id: I31c6df3aa420e1520a5e912166a70653770d6226