rust: regulator: relax a few constraints on

Merge series from Daniel Almeida <daniel.almeida@collabora.com>:

This series implement two related changes to address a bit of an oversight
on my end on the initial patch for the Regulator abstraction. Note that
this is not a fix, as it just relaxes the constraints on the previous code
as it is safe to do so.

Patch 1 removes some needless &mut self for functions that already provide
their own locking on the C side.

Patch 2 implements Send and Sync. In particular, there is no reason for
Regulator<T> not to be Send, and as discussed above, it is naturally Sync.