regulator: print symbolic errors in kernel messages

Change all error-printing messages to include error name via %pe instead
of numeric error or nothing.

Signed-off-by: Michał Mirosław <mirq-linux@rere.qmqm.pl>
Link: https://lore.kernel.org/r/1dcf25f39188882eb56918a9aa281ab17b792aa5.1601155770.git.mirq-linux@rere.qmqm.pl
Signed-off-by: Mark Brown <broonie@kernel.org>