power: regulator: Handle autoset in regulators_enable_boot_on()

With a recent change, regulators_enable_boot_on() returns an error if a
regulator is already set. Check for and handle this situation.

Fixes: d99fb64a power: regulator: Only run autoset once for each regulator
Reviewed-by: Jonas Karlman <jonas@kwiboo.se>
Reviewed-by: Quentin Schulz <quentin.schulz@cherry.de>

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Quentin Schulz <quentin.schulz@cherry.de>
Reviewed-by: Jonas Karlman <jonas@kwiboo.se>