si4708: enable fullscan

With real mute support, it should work without audible glitches.

Change-Id: Ibc11a92a67091d377c01d0740bb5dbad19eff439
si4708: cleanup

Consistent naming, reduced logspam, et cetera. No functional changes.

Change-Id: Id5a0360f9193133dfffc24ab172b0ccaec346fce