rust: regulator: remove needless &mut from member functions

rust: regulator: remove needless &mut from member functions

Regulator functions like "regulator_enable()" and "regulator_disable()"
already provide their own locking through "regulator_lock_dependent()", so
we can safely call the Rust API with a shared reference.

This was already the case with Regulator::set_voltage() on the Rust side,
but it was forgotten for Regulator::enable() and Regulator::disable().

Signed-off-by: Daniel Almeida <daniel.almeida@collabora.com>
Link: https://patch.msgid.link/20250729-regulator-send-sync-v1-1-8bcbd546b940@collabora.com
Reviewed-by: Alexandre Courbot <acourbot@nvidia.com>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: Mark Brown <broonie@kernel.org>