regulator: remove superfluous lock in regulator_resolve_coupling()

The code modifies rdev, but locks c_rdev instead. Remove the lock
as this is held together by regulator_list_mutex taken in the caller.

Fixes: f9503385 ("regulator: core: Mutually resolve regulators coupling")
Signed-off-by: Michał Mirosław <mirq-linux@rere.qmqm.pl>
Reviewed-by: Dmitry Osipenko <digetx@gmail.com>
Link: https://lore.kernel.org/r/25eb81cefb37a646f3e44eaaf1d8ae8881cfde52.1597195321.git.mirq-linux@rere.qmqm.pl


Signed-off-by: Mark Brown <broonie@kernel.org>