twsi: some variants clear interrupt flag by writing 0, others by writing 1

Make that distinction more explicit and regular in the code.
The difference in behavior is documented in the respective datasheets.

Previously, the code handled the distinction by writing the control
register multiple times where at least one write was zero and another
was one.

This can be considered a follow-up to r363021.

Reviewed by: manu
MFC after: 4 weeks
Differential Revision: https://reviews.freebsd.org/D26308