regulator: don't require mutex for regulator_notifier_call_chain()

Since 3801b86a ("regulator: Refactor supply implementation
to work as regular consumers") we no longer cascade notifications
and so notifier head's built-in rwsem is enough to protect the
notifier chain. Remove the requirement to fix one case where
rdev->mutex might be forced to be taken recursively.

Signed-off-by: Michał Mirosław <mirq-linux@rere.qmqm.pl>
Reviewed-by: Dmitry Osipenko <digetx@gmail.com>
Link: https://lore.kernel.org/r/5a0da9017c69a4dbc3f9b50f44476fce80a73387.1597032945.git.mirq-linux@rere.qmqm.pl


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