osmux: Avoid initing output without enabling osmux

Otherwise we end up in a weird state where we have timers set up but
osmux is still flagged as not enabled.

Cherry-picked from openbsc cad739d2386640a68c24e3d470ddacdcaf377561.
Change-Id: I0a334842463d311bc80a980e60fb702a0a9ad610
1 file changed