regulator: Allow autosetting fixed regulators

Fixed regulators don't have a set_value method. Therefore, trying to
set their value will always return -ENOSYS.

Signed-off-by: Sven Schwermer <sven@svenschwermer.de>
Cc: Jaehoon Chung <jh80.chung@samsung.com>
Cc: Peng Fan <peng.fan@nxp.com>
Reviewed-by: Simon Glass <sjg@chromium.org>