twsi: protect interaction between twsi_transfer and twsi_intr

All accesses to softc are now done under a mutex to prevent data races
between the open context and the interrupt handler.
Additionally, the wait time in twsi_transfer is bounded now.
Previously we could get stuck there forever if an interrupt got lost.

(cherry picked from commit 69cfa60ddd4df814b507699f9bb7253ece054f40)