regulator/driver: be more specific in nanodoc for is_enabled

Document the possibility that is_enabled may also return with negative
errorcodes.

Signed-off-by: Wolfram Sang <w.sang@pengutronix.de>
Acked-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Liam Girdwood <lrg@slimlogic.co.uk>