adc: Allow use of adc with DM_REGULATOR disabled

When DM_REGULATOR=n is used the device_get_supply_regulator() function
always return -ENOSYS.

Change to treat missing support for regulators as a missing optional
vdd/vss-supply regulator to reduce error messages being logged.

Signed-off-by: Jonas Karlman <jonas@kwiboo.se>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>
1 file changed